Credits
Issue an account credit and apply it as a deduction on the next invoice.
A Credit is goodwill granted to an account: an amount knocked off what it will owe, separate from any payment made. It belongs to the account, not to one student, so on a shared or family account it covers whatever you bill that account next.
A credit is not the same as an account being “in credit”. An account is in credit when it has paid more than it owes; a credit you issue is goodwill on top of that. The two are tracked separately. A credit never turns into cash; returning money an account has actually paid is a refund.