If you have a situation is which a patient was mistakenly added to the wrong customer some time ago and you need to currect the situation and keep the patient's medical history but move all financial transations involving the patient to the correct owner, then do as follows:
1. Reverse any transactions on the OLD Customer that involved the patient to be transferred....even if they included another pet from that customer's file.
2. Reverse any payments associated with those invoices. If payments cannot be reversed that match the invoice total - create a REFUND of the correct payment type.
3. At this point the OLD customer balance should be the same before you started.
4. Transfer the Patient to the NEW(Correct) Customer.
5. At this point you have the history, patient and customer correct - you just need to fix the transaction history.
6. If invoices that you reversed contained transactions that were still relevant to the OLD Customer, recreate these invoices and pay them using the same payment type you reversed.
7. In the new Customer recreate the relevant invoice items you reversed from the old customer and pay them using the same payment method you either created the refund for OR reversed in step 2. Adjust line item costs to be the same (if prices have risen).
8. Make notes on each new invoice about the actual date the transaction took place on - do the same for payments.
9. At this point both client Balances would be in theory either zero or reflect the balance before you started the process.
10. Run a balance check over each customer.
11. The TILL for this day will have a lot of transactions that don't really exist - but the reversals and payments should balance each other out, so the nett effect on the daily total should be zero.