There is not a built in way to move credit from one account to another. However, if you want a 'paper trail' for this, you can do the following:
- Make sure you have a payment type such as "Write Off" that has the option "Exclude from Income Calculations on Sales Report" enabled (Menu > Settings > Billing > Payments > Add New Payment Option).
- Go to the client that has the credit and refund the credit using the "Write Off" payment type.
- Go to the client that will receive the credit and make a deposit using the payment type of "Write Off".