diff --git a/libgnucash/engine/gncInvoice.c b/libgnucash/engine/gncInvoice.c
index 616ef1b1a2..60f643a7b9 100644
--- a/libgnucash/engine/gncInvoice.c
+++ b/libgnucash/engine/gncInvoice.c
@@ -1509,8 +1509,11 @@ Transaction * gncInvoicePostToAccount (GncInvoice *invoice, Account *acc,
     total = gncInvoiceGetTotal (invoice);
     taxes = gncInvoiceGetTotalTaxList (invoice);
     /* The two functions above return signs relative to the document
-     * We need to convert them to balance values before we can use them here */
-    if (is_cust_doc)
+     * We need to convert them to balance values before we can use them here
+     * Note the odd construct comparing two booleans is to xor them
+     * that is, only evaluate true if both are different.
+     */
+    if (is_cust_doc != is_cn)
     {
         GList *node;
         total = gnc_numeric_neg (total);
diff --git a/libgnucash/engine/test/utest-Invoice.c b/libgnucash/engine/test/utest-Invoice.c
index fe8eb7b769..e7726525ab 100644
--- a/libgnucash/engine/test/utest-Invoice.c
+++ b/libgnucash/engine/test/utest-Invoice.c
@@ -29,68 +29,190 @@
 static const gchar *suitename = "/engine/gncInvoice";
 void test_suite_gncInvoice ( void );
 
+typedef struct
+{
+    gboolean is_cn;
+    gboolean is_cust_doc;
+    gnc_numeric quantity;
+    gnc_numeric price;
+} InvoiceData;
+
 typedef struct
 {
     QofBook *book;
     Account *account;
+    Account *account2;
     GncOwner owner;
     GncCustomer *customer;
+    GncVendor *vendor;
     gnc_commodity *commodity;
+
+    GncInvoice* invoice;
+    Transaction *trans;
 } Fixture;
 
 static void
 setup( Fixture *fixture, gconstpointer pData )
 {
+    const InvoiceData *data = (InvoiceData*) pData;
+
     fixture->book = qof_book_new();
 
     fixture->account = xaccMallocAccount(fixture->book);
+    fixture->account2 = xaccMallocAccount(fixture->book);
     fixture->commodity = gnc_commodity_new(fixture->book, "foo", "bar", "xy", "xy", 100);
     xaccAccountSetCommodity(fixture->account, fixture->commodity);
+    xaccAccountSetCommodity(fixture->account2, fixture->commodity);
 
-    fixture->customer = gncCustomerCreate(fixture->book);
-    gncOwnerInitCustomer(&fixture->owner, fixture->customer);
+    if (data->is_cust_doc)
+    {
+        fixture->customer = gncCustomerCreate(fixture->book);
+        gncOwnerInitCustomer(&fixture->owner, fixture->customer);
+    }
+    else
+    {
+        fixture->vendor = gncVendorCreate(fixture->book);
+        gncOwnerInitVendor(&fixture->owner, fixture->vendor);
+    }
+
+    fixture->invoice = gncInvoiceCreate(fixture->book);
 }
 
 static void
 teardown( Fixture *fixture, gconstpointer pData )
 {
-    gncCustomerBeginEdit(fixture->customer);
-    gncCustomerDestroy(fixture->customer);
+    const InvoiceData *data = (InvoiceData*) pData;
+
+    gncInvoiceBeginEdit(fixture->invoice);
+    gncInvoiceDestroy(fixture->invoice);
+
+    if (data->is_cust_doc)
+    {
+        gncCustomerBeginEdit(fixture->customer);
+        gncCustomerDestroy(fixture->customer);
+    }
+    else
+    {
+        gncVendorBeginEdit(fixture->vendor);
+        gncVendorDestroy(fixture->vendor);
+    }
 
     xaccAccountBeginEdit(fixture->account);
     xaccAccountDestroy(fixture->account);
+    xaccAccountBeginEdit(fixture->account2);
+    xaccAccountDestroy(fixture->account2);
     gnc_commodity_destroy(fixture->commodity);
 
     qof_book_destroy( fixture->book );
+};
+
+static void
+setup_with_invoice( Fixture *fixture, gconstpointer pData )
+{
+    const InvoiceData *data = (InvoiceData*) pData;
+
+    time64 ts1 = gnc_time(NULL);
+    time64 ts2 = ts1;
+    const char *desc = "Test description";
+    GncEntry *entry = NULL;
+
+    setup(fixture, pData);
+
+    fixture->invoice = gncInvoiceCreate(fixture->book);
+    gncInvoiceSetCurrency(fixture->invoice, fixture->commodity);
+    gncInvoiceSetOwner(fixture->invoice, &fixture->owner);
+
+    entry = gncEntryCreate(fixture->book);
+    gncEntrySetDate (entry, ts1);
+    gncEntrySetDateEntered (entry, ts1);
+    gncEntrySetDescription (entry, desc);
+    gncEntrySetDocQuantity (entry, data->quantity, data->is_cn);
+
+    if (data->is_cust_doc)
+    {
+        gncEntrySetInvAccount(entry, fixture->account);
+        gncInvoiceAddEntry (fixture->invoice, entry);
+    }
+    else
+    {
+        gncEntrySetBillAccount(entry, fixture->account);
+        gncBillAddEntry(fixture->invoice, entry);
+    }
+
+    fixture->trans = gncInvoicePostToAccount(fixture->invoice, fixture->account2, ts1, ts2, "memo", TRUE, FALSE);
+}
+
+static void
+teardown_with_invoice( Fixture *fixture, gconstpointer pData )
+{
+    gncInvoiceUnpost(fixture->invoice, TRUE);
+    gncInvoiceRemoveEntries (fixture->invoice);
+
+    teardown(fixture, pData);
 }
 
 
 static void
 test_invoice_post ( Fixture *fixture, gconstpointer pData )
 {
-    GncInvoice *invoice = gncInvoiceCreate(fixture->book);
     time64 ts1 = gnc_time(NULL);
     time64 ts2 = ts1;
-    g_assert(invoice);
-    g_assert(!gncInvoiceGetIsCreditNote(invoice));
-    g_assert(gncInvoiceGetActive(invoice));
-    g_assert(gncInvoiceGetPostedAcc(invoice) == NULL);
+    g_assert(fixture->invoice);
+    g_assert(!gncInvoiceGetIsCreditNote(fixture->invoice));
+    g_assert(gncInvoiceGetActive(fixture->invoice));
+    g_assert(gncInvoiceGetPostedAcc(fixture->invoice) == NULL);
 
-    gncInvoiceSetCurrency(invoice, fixture->commodity);
+    gncInvoiceSetCurrency(fixture->invoice, fixture->commodity);
 
-    gncInvoiceSetOwner(invoice, &fixture->owner);
+    gncInvoiceSetOwner(fixture->invoice, &fixture->owner);
 
     g_test_message( "Will now post the invoice" );
-    g_assert(!gncInvoiceIsPosted(invoice));
-    gncInvoicePostToAccount(invoice, fixture->account, ts1, ts2, "memo", TRUE, FALSE);
-    g_assert(gncInvoiceIsPosted(invoice));
+    g_assert(!gncInvoiceIsPosted(fixture->invoice));
+    gncInvoicePostToAccount(fixture->invoice, fixture->account, ts1, ts2, "memo", TRUE, FALSE);
+    g_assert(gncInvoiceIsPosted(fixture->invoice));
+
+    gncInvoiceUnpost(fixture->invoice, TRUE);
+    g_assert(!gncInvoiceIsPosted(fixture->invoice));
+}
+
+static void
+test_invoice_posted_trans ( Fixture *fixture, gconstpointer pData )
+{
+    const InvoiceData *data = (InvoiceData*) pData;
 
-    gncInvoiceUnpost(invoice, TRUE);
-    g_assert(!gncInvoiceIsPosted(invoice));
+    gnc_numeric total = gncInvoiceGetTotal(fixture->invoice);
+    gnc_numeric acct_balance, acct2_balance;
+
+    g_assert (1 == xaccAccountCountSplits (fixture->account, FALSE));
+    g_assert (1 == xaccAccountCountSplits (fixture->account2, FALSE));
+
+    acct_balance = xaccAccountGetBalance(fixture->account);
+    acct2_balance = xaccAccountGetBalance(fixture->account2);
+
+    // Handle sign reversals (document values vs balance values)
+    if (data->is_cn != !data->is_cust_doc)
+    {
+        g_assert (gnc_numeric_equal (gnc_numeric_neg(acct_balance), total));
+        g_assert (gnc_numeric_equal (acct2_balance, total));
+    }
+    else
+    {
+        g_assert (gnc_numeric_equal (acct_balance, total));
+        g_assert (gnc_numeric_equal (gnc_numeric_neg(acct2_balance), total));
+    }
 }
 
 void
 test_suite_gncInvoice ( void )
 {
-    GNC_TEST_ADD( suitename, "post", Fixture, NULL, setup, test_invoice_post, teardown );
+    InvoiceData pData = { FALSE, FALSE, { 1000, 100 }, { 2000, 100 } };  // Vendor bill
+    GNC_TEST_ADD( suitename, "post/unpost", Fixture, &pData, setup, test_invoice_post, teardown );
+
+    GNC_TEST_ADD( suitename, "post trans - vendor bill", Fixture, &pData, setup_with_invoice, test_invoice_posted_trans, teardown_with_invoice );
+    pData.is_cn = TRUE;   // Vendor credit note
+    GNC_TEST_ADD( suitename, "post trans - vendor credit note", Fixture, &pData, setup_with_invoice, test_invoice_posted_trans, teardown_with_invoice );
+    pData.is_cust_doc = TRUE;   // Customer credit note
+    GNC_TEST_ADD( suitename, "post trans - customer creditnote", Fixture, &pData, setup_with_invoice, test_invoice_posted_trans, teardown_with_invoice );
+    pData.is_cn = FALSE;   // Customer invoice
+    GNC_TEST_ADD( suitename, "post trans - customer invoice", Fixture, &pData, setup_with_invoice, test_invoice_posted_trans, teardown_with_invoice );
 }