As you can see the grouping is on invoice date and payment
according to your query
GROUP BY tblClient.Client_Name, tblCustomer.Customer_Name, tblInvoice.Invoice_Amount, tblInvoice.Discount
you are grouping on name, amount and discount. And you are summing the date
Sum(tblPmts.Pmt_Date) AS SumOfPmt_Date
suggest try this
Code:
SELECT tblClient.Client_Name, tblCustomer.Customer_Name, tblPmts.Pmt_Date, tblInvoice.Invoice_Amount, Sum(tblPmts.Pmt_Amount) AS SumOfPmt_Amount, tblInvoice.Discount
FROM (tblClient INNER JOIN (tblCustomer INNER JOIN tblInvoice ON tblCustomer.CustomerID_Pk = tblInvoice.CustomerID_Fk) ON tblClient.ClientID_Pk = tblInvoice.ClientID_Fk) LEFT JOIN tblPmts ON tblInvoice.InvoiceID_Pk = tblPmts.InvoiceID_Fk
GROUP BY tblClient.Client_Name, tblCustomer.Customer_Name, tblPmts.Pmt_Date, tblInvoice.Invoice_Amount, tblInvoice.Discount
HAVING (((tblClient.Client_Name)=[Forms]![frmBalance].[cboClientName]));