Give a subscriber credit

Puts credit on the subscriber's account with you. It comes off their next invoice, reducing what their card is charged; no money moves out, and no receipt already issued changes. The credit belongs to the CUSTOMER, so if the same buyer has two subscriptions with you in the same currency it goes to whichever invoice falls due first. The amount is positive integer minor units and may cover the whole of the next invoice — at most its amount due. A credit that covers it in full charges the card nothing, and that month is still recorded as a sale of zero with an invoice and a receipt saying the credit covered it. More than the amount due is refused, and the error names the largest amount you can give. The currency is the subscription's; there is no currency field. Only an owner or admin may do this, from a signed-in session: an API key cannot, because the credit must name the person who gave it.