Credit Card Swipe Interference
Interesting, it sounds like your laser scanner is causing noise on the power cable which then interferes with the POS machines operation.  This is probably due to a poorly designed power supply in either of the machines.

If you can't use just a different outlet, then it might be worth trying to put a ferrous nugget around the power cable of the POS machine before you spend the money to put in a new outlet.

A ferrous nugget serves to filter out noise on power cables and many products have them.