I would guess what happens is actual money does not change hands when a purchase is made. Instead, Apple keeps records of what it owes each developer, and at s

I would guess what happens is actual money does not change hands when a purchase is made. Instead, Apple keeps records of what it owes each developer, and at some later date (likely monthly), it makes one big payment to the developer. Thus at purchase time, Apple enters +$70 in the records it keeps of what it owes a developer. And at refund time, it enters -$100. So the net effect of a purchase and then refund is +$70 + -$100 = -$30.