Thank you Paul for this tip...
If you want to manually add a "credit" or a "refund-credit" into a client's account that they can use toward a bill or a future purchase -AND- that will not automatically be applied to any existing invoices then you should:
STEP 1 - Go to Settings > Company > Billing/Payment. Uncheck and save "Automatically apply loose credits". You may need to do this under Settings > Company > Client Groups also, if your Client Group settings are overriding your Company Settings.
STEP 2 - On the client profile page, under "Account Actions" - - > click "Record Payment" - - > Enter the amount - - > select "In House Credit" - - > and DO NOT select any existing invoices that may be displayed.