Let's assume user checkout 150$ of all items. Right now he has special code with 50$ money on it. I would like to make him possible to provide this 50$ code and to do additional payment for rest of it, means 100$. At the end user has to be invoiced for 150$ because this 50$ wasn't a gift, it was something like virtual prepaid card with this money on it.
When user has provided the code, custom api call is triggered to check if this code is valid for this money for this buyer.
When user confirm, custom api call is triggered to approve transaction.
When user reject, custom api call is triggered to not cancel this virtual card and whole order in Shopify is going to be cancelled.
Code is generated in special android/ios/web app from collected by user money on his account (separated from Shopify).
On all payment methods should be another extra option, pay by our virtual card.