Urgent invoice adjustments with credit payment!!!

I created an invoice for an customer eg. $3000. and this customer has some credit in his account. eg.$4000. He used his credit to pay for the invoice but later I found that the invoice amount is $2000 instead of $3000.

I don't know how and where to adjust the invoice since I cannot find the payment record anywhere. I cannot delete this payment and adjust the invoice amount!!!!!!

please help