How do I transfer a credit balance from an account to pay off the balance from one of the other dentists within that practice?

The method is very similar to that described in this Help Desk article Record payment receipts from a Principal Account

In the account from which you want to transfer credit, Make a Refund/Debit choosing a method of say 'Debit Adjustment'
In the account which is to receive the credit, make a 'Receive Payment/Give Credit' credit, choosing 'Credit Adjustment'. Allow LabManager to auto-allocate this credit.

Use the comment and/or ref boxes appropriately to make it easy to trace at a later date.