Using store loyalty points to pay off a debt

I owe money to a person. In lieu of direct payment, I occasionally do some shopping on the creditor's behalf and thus reduce the amount I owe. What happens if I use my store benefit/loyalty points when shopping?

Example: total amount purchased R500. I used my loyalty point balance of R100 and total amount paid was R400. I occasionally also purchase items from this store for my personal use. Do I reduce the amount I owe by R500 or R400? In essence - who is entitled to the benefit/cash back reward from the store?