diff --git a/Fuchs.Tests/InvoiceRegisterSqlTests.cs b/Fuchs.Tests/InvoiceRegisterSqlTests.cs
new file mode 100644
index 0000000..44a5012
--- /dev/null
+++ b/Fuchs.Tests/InvoiceRegisterSqlTests.cs
@@ -0,0 +1,62 @@
+using System;
+using Fuchs.Services;
+using Xunit;
+
+namespace Fuchs.Tests;
+
+/// <summary>
+/// The invoice header save batch must stop as soon as fds__createInvoice / fds__setInvoice fails:
+/// previously a parameter conversion error left @Id NULL and the batch went on to
+/// fds__createInvoice_Details, whose "Cannot insert NULL into InvId" buried the real error.
+/// </summary>
+public class InvoiceRegisterSqlTests
+{
+    [Theory]
+    [InlineData(false, "[dbo].[fds__createInvoice] ", "fds__createInvoice did not return")]
+    [InlineData(true, "[dbo].[fds__setInvoice] ", "fds__setInvoice did not return")]
+    public void BuildRegisterInvoiceSql_GuardSitsBetweenHeaderProcAndDetails(bool change, string headerProc, string guardText)
+    {
+        string sql = InvoiceService.BuildRegisterInvoiceSql(change, hasProvisionLocation: true);
+
+        int header  = sql.IndexOf(headerProc, StringComparison.Ordinal);
+        int guard   = sql.IndexOf("IF @Id IS NULL THROW", StringComparison.Ordinal);
+        int details = sql.IndexOf("[dbo].[fds__createInvoice_Details]", StringComparison.Ordinal);
+        int update  = sql.IndexOf("UPDATE [dbo].[fds__invoices]", StringComparison.Ordinal);
+
+        Assert.True(header >= 0, "header proc missing");
+        Assert.True(header < guard && guard < details && details < update, sql);
+        Assert.Contains(guardText, sql);
+    }
+
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void BuildRegisterInvoiceSql_ExactlyOneHeaderProcPerMode(bool change)
+    {
+        string sql = InvoiceService.BuildRegisterInvoiceSql(change, hasProvisionLocation: false);
+
+        Assert.Equal(change, sql.Contains("[dbo].[fds__setInvoice] @InvId,"));
+        Assert.Equal(!change, sql.Contains("[dbo].[fds__createInvoice] @InvoiceType,"));
+        Assert.Equal(change, sql.Contains("@InvId"));
+    }
+
+    [Theory]
+    [InlineData(false, false)]
+    [InlineData(true, true)]
+    public void BuildRegisterInvoiceSql_ProvisionLocationUpdateOnlyWhenPresent(bool hasProvisionLocation, bool expected)
+    {
+        string sql = InvoiceService.BuildRegisterInvoiceSql(change: true, hasProvisionLocation);
+
+        Assert.Equal(expected, sql.Contains("@ProvisionLocation"));
+    }
+
+    [Fact]
+    public void BuildRegisterInvoiceSql_EveryStatementIsSemicolonTerminated()
+    {
+        // THROW requires the preceding statement to be terminated with ';'.
+        string sql = InvoiceService.BuildRegisterInvoiceSql(change: true, hasProvisionLocation: true);
+
+        foreach (var line in sql.Split('\n'))
+            Assert.EndsWith(";", line.TrimEnd());
+    }
+}
diff --git a/Fuchs.Tests/SqlNumberParameterTests.cs b/Fuchs.Tests/SqlNumberParameterTests.cs
new file mode 100644
index 0000000..32576dc
--- /dev/null
+++ b/Fuchs.Tests/SqlNumberParameterTests.cs
@@ -0,0 +1,107 @@
+using System;
+using System.Globalization;
+using System.Linq;
+using Fuchs.intranet;
+using Newtonsoft.Json.Linq;
+using Xunit;
+using static OCORE.SQL.sql;
+
+namespace Fuchs.Tests;
+
+/// <summary>
+/// Regression tests for the OCORE string → number SQL parameter helpers. Saving a Stornorechnung
+/// failed in production with "Error converting data type float to numeric": the negative totals
+/// did not parse in the first (sign-less) attempt and fell back to the server's de-DE culture,
+/// where "." is a group separator — "-19.95" became -1995 and JS float noise
+/// ("-1234.5600000000002") overflowed numeric(10,3), so fds__setInvoice never ran.
+/// Every test runs under de-DE, the production server culture.
+/// </summary>
+public class SqlNumberParameterTests : IDisposable
+{
+    private readonly CultureInfo _culture = CultureInfo.CurrentCulture;
+
+    public SqlNumberParameterTests() => CultureInfo.CurrentCulture = new CultureInfo("de-DE");
+
+    public void Dispose() => CultureInfo.CurrentCulture = _culture;
+
+    [Theory]
+    [InlineData("-19.95", -19.95)]
+    [InlineData("-1234.56", -1234.56)]
+    [InlineData("-1234.5600000000002", -1234.56)]
+    [InlineData("-0.5", -0.5)]
+    [InlineData("1234.56", 1234.56)]
+    [InlineData("1234.5600000000002", 1234.56)]
+    [InlineData("-1,5", -1.5)]
+    [InlineData("-1.234,56", -1234.56)]
+    [InlineData("1.234,56", 1234.56)]
+    [InlineData("1,234.56", 1234.56)]
+    [InlineData(" -42 ", -42)]
+    [InlineData("-1.5E-05", -0.000015)]
+    [InlineData("0", 0)]
+    public void SQL_Float_StringValue_ParsesSignedAndCultureFormattedNumbers(string input, double expected)
+    {
+        var p = SQL_Float("@v", stringvalue: input);
+
+        var value = Assert.IsType<float>(p.Value);
+        Assert.Equal(expected, value, 0.01);
+    }
+
+    [Theory]
+    [InlineData("-19.95", -19.95)]
+    [InlineData("-1234.56", -1234.56)]
+    [InlineData("-1.234,56", -1234.56)]
+    [InlineData("1.234,56", 1234.56)]
+    [InlineData("-1,5", -1.5)]
+    public void SQL_Decimal_StringValue_ParsesSignedAndCultureFormattedNumbers(string input, double expected)
+    {
+        var p = SQL_Decimal("@v", input, precision: 10, scale: 3);
+
+        var value = Assert.IsType<decimal>(p.Value);
+        Assert.Equal((decimal)expected, value);
+    }
+
+    [Theory]
+    [InlineData("")]
+    [InlineData("NaN")]
+    [InlineData("abc")]
+    [InlineData("--5")]
+    public void SQL_Float_StringValue_InvalidOrEmpty_IsDbNull(string input)
+    {
+        var p = SQL_Float("@v", stringvalue: input);
+
+        Assert.Equal(DBNull.Value, p.Value);
+    }
+
+    [Theory]
+    [InlineData("")]
+    [InlineData("NaN")]
+    [InlineData("abc")]
+    public void SQL_Decimal_StringValue_InvalidOrEmpty_IsDbNull(string input)
+    {
+        var p = SQL_Decimal("@v", input);
+
+        Assert.Equal(DBNull.Value, p.Value);
+    }
+
+    [Fact]
+    public void BuildInvoiceParams_StornoNegativeTotals_KeepsTheirValueAndFitTheProcNumerics()
+    {
+        // Shape of a Stornorechnung post: negated totals with JS float noise.
+        var jo = JObject.Parse(
+            "{ \"admin\": { \"type\": \"S\", \"customerid\": \"7\" }, " +
+            "  \"new\":   { \"title\": \"Stornorechnung\", \"total_gross\": \"-1469.1400000000003\", \"total_net\": \"-1234.5600000000002\" }, " +
+            "  \"sms\":   { \"vat\": { \"19,0%\": -234.58000000000004 }, \"tscn\": \"-1234.56\", \"tscvat\": \"-234.58\" } }");
+
+        var pl = new FdsInvoiceData(jo).BuildInvoiceParams(change: true, invId: "abc");
+        double Val(string n) => Convert.ToDouble(Assert.IsType<float>(pl.Single(p => p.ParameterName == n).Value));
+
+        Assert.Equal(-1469.14, Val("@InvoiceBalance"), 0.01);
+        Assert.Equal(-1234.56, Val("@InvoiceBalance_net"), 0.01);
+        Assert.Equal(-234.58, Val("@InvoiceVAT_net1"), 0.01);
+        Assert.Equal(-1234.56, Val("@InvoiceService_net"), 0.01);
+        Assert.Equal(-234.58, Val("@InvoiceService_VAT"), 0.01);
+        // numeric(10,3) / numeric(10,2) in fds__setInvoice / fds__createInvoice_Details.
+        Assert.All(new[] { "@InvoiceBalance", "@InvoiceBalance_net", "@InvoiceVAT_net1", "@InvoiceService_net", "@InvoiceService_VAT" },
+            n => Assert.InRange(Math.Abs(Val(n)), 0, 9_999_999.99));
+    }
+}
diff --git a/Fuchs/Services/InvoiceService.cs b/Fuchs/Services/InvoiceService.cs
index 1bd8da3..a67c111 100644
--- a/Fuchs/Services/InvoiceService.cs
+++ b/Fuchs/Services/InvoiceService.cs
@@ -151,6 +151,27 @@ public class InvoiceService : IInvoiceService
         return blocks;
     }
 
+    /// <summary>
+    /// The header save batch for <see cref="RegisterInvoiceAsync"/>. If the create/set proc fails
+    /// (e.g. a parameter conversion error) the batch would otherwise carry on with a NULL
+    /// <c>@Id</c> and fail again in <c>fds__createInvoice_Details</c> ("Cannot insert NULL into
+    /// InvId"), burying the real error; the guard stops the batch right after the first failure.
+    /// </summary>
+    internal static string BuildRegisterInvoiceSql(bool change, bool hasProvisionLocation)
+    {
+        var sqlParts = new List<string> { "DECLARE @Id varchar(10);" };
+        sqlParts.Add(change
+            ? "EXECUTE [dbo].[fds__setInvoice] @InvId, @InvoiceType, @InvoiceTitle, @InvoiceBalance, @InvoiceBalance_net, @InvoiceVAT_net1, @InvoiceVAT_1, @PaymentTerm, @CustomerId, @SendToAddress, @SendToEmail, @SendToEmailCc, @ProvisionPeriod, @CustomValues, @authuser, @Id OUTPUT, @SendToAddressJson;"
+            : "EXECUTE [dbo].[fds__createInvoice] @InvoiceType, @InvoiceTitle, @InvoiceBalance, @InvoiceBalance_net, @InvoiceVAT_net1, @InvoiceVAT_1, @PaymentTerm, @CustomerId, @SendToAddress, @SendToEmail, @SendToEmailCc, @ProvisionPeriod, @CustomValues, @authuser, @Id OUTPUT, @SendToAddressJson;");
+        sqlParts.Add(change
+            ? "IF @Id IS NULL THROW 50001, N'fds__setInvoice did not return an invoice id - invoice header not saved.', 1;"
+            : "IF @Id IS NULL THROW 50001, N'fds__createInvoice did not return an invoice id - invoice header not saved.', 1;");
+        sqlParts.Add("EXECUTE [dbo].[fds__createInvoice_Details] @Id, @InvoiceService_net, @InvoiceService_VAT, @InvoiceOptions, @authuser;");
+        if (hasProvisionLocation)
+            sqlParts.Add("UPDATE [dbo].[fds__invoices] SET [ProvisionLocation] = LEFT(@ProvisionLocation,1000) WHERE [Id] = @Id AND [isFinal] = 0;");
+        return string.Join("\n", sqlParts);
+    }
+
     public async Task<FdsInvoiceData> RegisterInvoiceAsync(FdsInvoiceData invoice, bool change, string invId,
         string userAccountId, DatabaseSecurity dbSec)
     {
@@ -160,26 +181,13 @@ public class InvoiceService : IInvoiceService
         var pl = new List<SqlParameter> { SQL_VarChar("@authuser", userAccountId) };
         pl.AddRange(invoice.BuildInvoiceParams(change, invId));
 
-        var sqlParts = new List<string> { "DECLARE @Id varchar(10);" };
-        if (!change)
-        {
-            sqlParts.Add("EXECUTE [dbo].[fds__createInvoice] @InvoiceType, @InvoiceTitle, @InvoiceBalance, @InvoiceBalance_net, @InvoiceVAT_net1, @InvoiceVAT_1, @PaymentTerm, @CustomerId, @SendToAddress, @SendToEmail, @SendToEmailCc, @ProvisionPeriod, @CustomValues, @authuser, @Id OUTPUT, @SendToAddressJson;");
-            sqlParts.Add("EXECUTE [dbo].[fds__createInvoice_Details] @Id, @InvoiceService_net, @InvoiceService_VAT, @InvoiceOptions, @authuser;");
-        }
-        else
-        {
-            pl.Add(SQL_VarChar("@InvId", invId));
-            sqlParts.Add("EXECUTE [dbo].[fds__setInvoice] @InvId, @InvoiceType, @InvoiceTitle, @InvoiceBalance, @InvoiceBalance_net, @InvoiceVAT_net1, @InvoiceVAT_1, @PaymentTerm, @CustomerId, @SendToAddress, @SendToEmail, @SendToEmailCc, @ProvisionPeriod, @CustomValues, @authuser, @Id OUTPUT, @SendToAddressJson;");
-            sqlParts.Add("EXECUTE [dbo].[fds__createInvoice_Details] @Id, @InvoiceService_net, @InvoiceService_VAT, @InvoiceOptions, @authuser;");
-        }
-        if (invoice.RawProvisionLocation.Length > 0)
-        {
+        if (change) pl.Add(SQL_VarChar("@InvId", invId));
+        bool hasProvisionLocation = invoice.RawProvisionLocation.Length > 0;
+        if (hasProvisionLocation)
             pl.Add(SQL_NVarChar("@ProvisionLocation",
                 string.Join("\n", invoice.RawProvisionLocation).LeftToFirst("<!--", emptyIfNotFound: false)));
-            sqlParts.Add("UPDATE [dbo].[fds__invoices] SET [ProvisionLocation] = LEFT(@ProvisionLocation,1000) WHERE [Id] = @Id AND [isFinal] = 0;");
-        }
 
-        var invdset = await getSQLDataSet_async(string.Join("\n", sqlParts),
+        var invdset = await getSQLDataSet_async(BuildRegisterInvoiceSql(change, hasProvisionLocation),
             Conn, pl, tablenames: new[] { "inv", "det" },
             Security: dbSec, options: new FIS_SQLOptions());
         if (!string.IsNullOrEmpty(invdset.Exception))
diff --git a/OCORE b/OCORE
index def034e..00a7923 160000
--- a/OCORE
+++ b/OCORE
@@ -1 +1 @@
-Subproject commit def034e155e5d28343d948c3daee2fee67707607
+Subproject commit 00a7923d0532085b2f0a244b446d4fb61e5f211e
diff --git a/hlp/2026-10-01 check negative invoice amounts.sql b/hlp/2026-10-01 check negative invoice amounts.sql
new file mode 100644
index 0000000..f92fc82
--- /dev/null
+++ b/hlp/2026-10-01 check negative invoice amounts.sql	
@@ -0,0 +1,29 @@
+-- Read-only check for amounts mangled by the OCORE SQL_Float sign bug (fixed 2026-10-01).
+-- Before the fix, a negative amount posted as "-19.95" was parsed with the server's de-DE culture
+-- ("." = thousands separator) and stored scaled up, e.g. -19.95 -> -1995. Positive amounts were fine.
+-- Affected: fds__invoices header totals (Stornos / credits) and fds__invoice_details service totals
+-- written through fds__setInvoice / fds__createInvoice / fds__createInvoice_Details, plus reminder
+-- amounts written by ReminderService. Line items (fds__invoice_items) were NOT affected.
+--
+-- Signal: the header net total disagrees with the sum of the invoice's own line items.
+
+-- 1) Invoices with a negative header total, compared against their persisted line items.
+SELECT  i.[Id], i.[InvoiceId], i.[InvoiceType], i.[IsFinal], i.[DateCreated], i.[DateModified],
+        i.[InvoiceBalance], i.[InvoiceBalance_net], i.[InvoiceVAT_net1],
+        d.[InvoiceService_net], d.[InvoiceService_VAT],
+        items_net = it.[items_net],
+        suspicious = CASE WHEN it.[items_net] IS NOT NULL
+                           AND ABS(i.[InvoiceBalance_net] - it.[items_net]) > 0.05 THEN 1 ELSE 0 END
+FROM    [dbo].[fds__invoices] i
+LEFT JOIN [dbo].[fds__invoice_details] d ON d.[InvId] = i.[Id]
+OUTER APPLY (SELECT [items_net] = SUM(x.[value_total])
+             FROM [dbo].[fds__invoice_items] x
+             WHERE x.[InvId] = i.[Id]) it
+WHERE   i.[InvoiceBalance] < 0 OR i.[InvoiceBalance_net] < 0
+     OR d.[InvoiceService_net] < 0 OR d.[InvoiceService_VAT] < 0
+ORDER BY suspicious DESC, i.[DateModified] DESC;
+
+-- 2) Reminders with a negative amount (same helper).
+SELECT  *
+FROM    [dbo].[fds__reminder]
+WHERE   [amount] < 0 OR [amount_payed] < 0;