@KJS 

You can obviously ignore the response from @MirriamM .  It's as though they didn't even read your question and it does nothing to address your issue.

In QB Desktop, you can't use A/R on two lines of a single journal entry (JE) so you'll need to use a temporary account and two JEs.  First, set up a bank account in QB called Clearing Account.  Then, make a JE that debits A/R for the customer that currently has the credit (customer 1)and credit the Clearing Account for the amount of the credit.  Then, make another JE that debits the Clearing Account and credits A/R for the customer (customer 2) that you want to move the credit to.  Finally, you need to clear the A/R debit and credit for customer 1 so go to Customers > Receive payment and apply the credit to the "invoice" created by the A/R debit.  Finally, you should have the credit available to use for customer 2.  You can mark the bank Clearing Account as inactive if you don't want to see it on your reports.  Hope that helps.