When a practice's data is migrated from your previous software supplier; the patients in credit will have an invoice in their accounts screen showing 'Opening Balance' and a credit indicating a sum of money.
In order to use this credit it needs to be turned into a payment, you can do this by;
Processing a payment of £0.00 with payment type 'other'
The practitioner is unimportant as the Income report will assign the payment to the practitioner who raised the charge for the work this payment is used to pay.
Click on the orange question mark by that payment to explain it against the transferred balance invoice.
Select the invoice with the negative value and enter that negative amount in the amount to explain the field.
At this point, you are essentially telling the system how much credit you need to create, from the unused credit invoice. This must be a negative amount or you will see a warning stating you are trying to explain more than the payment is worth.
Once saved, you will then see a positive payment amount to the value of the explanation.
This positive amount will be unexplained, so you can then use the orange question mark to explain this amount against an invoice in the usual way.
Alternatively, the amount will be automatically explained against the next raised invoice.