Spendable over several visits
A gift is rarely the exact price of a treatment. Someone buys a round number, the treatment costs something else, and the difference has to live somewhere.
So a voucher carries a balance rather than being used up all at once. Each part-spend is recorded against the visit it paid for, which means the balance is a sum your team can explain rather than a number that changed on its own. When it reaches zero, it is done.