Any transaction can be refunded either partially or fully. To refund only a part of the payment, use the parameter partial_amount.
Note: Max two refunds per transaction!
If a transaction is partially refunded, it's not possible to issue another partial refund, unless the amount matches exactly the remaining amount. In other words, a transaction can only be refunded twice, once with a partial amount and again with the full (remaining) amount. The refund cannot be split it three or more parts.