All perfect praise be to Allah The Lord of the Worlds and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
A prepaid shopping coupon represents a certain amount and, according to Sharia, it is considered a loan given by the Fund to the above stores. In other words, it is a debt. In light of Islamic Law, it is forbidden to sell a debt save on the spot, because selling it by installments leads to selling debt for debt.
Accordingly, it is forbidden to sell the above coupons by installments, even for their exact value, and if any addition was stipulated, then that leads to Riba (Usuary) since it becomes a loan conditioned with an addition.
In order for the above Fund to make profit, it should make a deal with the stores whereby it collects a certain percentage from the value of the purchases as Ji`alah(Conditional payment), then collects it from the staff members by installments and without any addition. And Allah knows best.