Hi,

I believe what you're suggesting here may already be possible. To add credit to a client's account and record it as a transaction, use the Billing > Transactions List > Add Transaction tab, and tick the "Add as Credit" checkbox: https://docs.whmcs.com/Transactions#Adding_a_Manual_Transaction

It is true that when the credit is then applied to an invoice, no transaction is created. Are you proposing a second transaction in that instance? Or would the above workflow meet your needs?