Credit balance allocation to more than one account

Hi,

I am in need of a little help/information.

I have a customer who has 3 accounts for different stores that we service - two of these accounts owe us money, but the third has a credit balance on their account.  This customer wants us to allocate the credit balance (which is in our last financial year) to the balances on the other two accounts.  What is the best way to do this?

TIA

Daphne