How can I move a credit from one client to another?

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:

1. 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).
2. Go to the client that has the credit and refund the credit using the "Write Off" payment type.
3. Go to the client that will receive the credit and make a deposit using the payment type of "Write Off".