Condition on the value selected from combox

[mmahmod]

Dear Gents

I have a main form for payment and sub form for invoices and the relationship is many to many
I would like in to chose the invoices from the combo box in the sub form based on condition
If the remaining amount of the invoice is less than the total payment , then accepted and updated the field of amount paid
If the remaining amount of the invoice is less than the remaining balance of the payment, then rejected it after show me a notification for that
Or if there a better way to handle it
I attached the file

Thanks in advance