Starts a refund for the specified invoice. amount is optional - if omitted, the full remaining invoice amount is refunded; if provided it must be > 0. The cap enforced is the invoice's REMAINING refundable balance (original amount minus any prior refunds already recorded for it), not simply the original invoice amount - a second partial refund can be rejected even though it's below the original total. The invoice must already be paid, with an authorised payment transaction on file, or this returns 422. If currency is not resolvable from the invoice, EUR is used as a fallback.
