Web development -- 2

I need some changes to an existing website. Prestashop developer needed I own a used book store. I am currently a brick and mortar store with a website. I give customers trade in credit for their used​ books and they can use trade​ in credit to purchase in my store for up to 50% off my sticker price. for example some one has $50 of trade in credit and want to purchase $2 book in my shop. We take $1 off the trade in credit and they pay $1 for the book. Then their available trade in credit would drop to $49 . If that same would then purchase $150 in books they would only receive $49 discount as that is all that is in their trade in credit account. There are certain skus that trade in credit and cannot be used so I would need to have an exclusion list as well for this skus . I would need an area that I could adjust the individual trade in credit amount as every customer has a different trade in credit amount and it changes when they bring in more books. I am currently using prestashop 1.6 but I am going to upgrade it to 1.7 in the next few months. I would like the module compatible with both version.