Is it permissible to sell a 20 Dinar credit allocated for goods in a shopping mall for 18 Dinars in cash, knowing that the credit cannot be cashed out from the mall, and the buyer fills up on goods worth 20 Dinars?

It appears that the credit is a gift, and selling it in this manner involves two issues: selling a gift before taking possession of it, which is permissable according to the preferred view. And selling it for less than its value, which is not prohibited unless it falls under usury, which is not the case here, as it is the sale of goods for a specific value in immediate cash. There remains the issue of the unknown nature of the specific goods, but it seems to be a minor, tolerable ambiguity that does not lead to dispute, unfairness, or harm. Therefore, knowing the value is sufficient to fulfill the condition of knowing the item being sold.