Sorry, can they (customers) demand it back?

I mean, once Starbucks have it, then the customers get it back via product (that has a margin included), or just leave it forever (free money!)

I have a firm "No vouchers" rule because of this, the vouchers in my part of the world inexplicably "expire" if not used within a certain amount of time, cannot be redeemed for cash, and will not be honoured if the business goes belly up