How to implement the idea of ​​paying for goods of a certain category from an internal account in Bitrix?

Question

1C-Bitrix

Please tell me how to implement a non-standard idea in Bitrix.
There is a catalog of goods, when we put the goods in the basket, a bonus is immediately credited (internal account), for this bonus, in a certain category of goods, we can collect goods for the amount of the bonus.
I absolutely do not understand how this can be done ... the goods have a price and in any case this price is added to the cart.
I do everything through the API. Edition Business.