Contact your bank initially and explain the problem to them.
Banks are surprisingly willing to help when it comes to credit card issues. If you know who issued the customers card (not just Mastercard) you could contact them although your bank should point you in the right direction.
"c > Hi there - my understanding is that the credit card information is masked / > encrypted within RMS, so the entire number cannot be retrieved (if you are > using the built-in EDC functionality) If you are using a third party tool > like pc charge or TPI they may be able to help you with this. You may have > to void the original transaction and settle both (the refund and the void), > so that you un-do the deposit the customer will receive as a result of > processing the transaction as a refund; if you can't get the customer to come > back to the store, you'll probably have to write off the item(s) they took > with them. > > "PDunn" wrote: > > > Yesterday, I erroneously rang up a sale as a return (MasterCard). I did not > > realize until the end of the day and am trying to contact the buyer. I have > > not settled transactions hoping to correct this problem. As I can only see > > the first/last four digits of his credit card number, I cannot make the > > correction manually at this time. Is the credit card data available in the > > database or is there a way for me to edit the transaction so that I can post > > it as a sale rather than return? Right now, he's due to receive a nice > > deposit from me!