You should not be storing calculations in tables. Where you away that day, or are they not teaching this most basic precept? 
In a case like this, aggregate (math) functions should be performed on the sum of all values in a field vs the sum of another field, and this should be done on a form. You would test if txtBalance exceeds txtPayment before processing the payment. Not sure why you'd want this rule anyway. Is it not possible that someone would accidentally make a payment twice or a payment that exceeds the balance? Surely I am not the only one who has ever done that.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.