If you have an account credit (maybe a redeemed gift certificate) applied to one customer, but they want to transfer it to someone else's account, this is how you would accomplish the exchange.
For our example we will be moving a $100 credit:
Go to the invoice that has a credit balance. Our example is pictured below.
In the Invoice Summary, there is a "pencil" icon next to the "Billed to" name. Use this to pull up and edit the "Billed to" customer. Once you have selected the customer to whom you want to move the credit, click Save.
You will now see this invoice under the new customer's account and the amount showing on their account balance.