We have no way to automatically create or issue "rebate" discounts like you describe, but you could simply extend that offer to buyers upon request, then when they contact you to take up the offer, just go into Seller Admin > Cart Discounts and create a unique Cart Discount Code for that buyer, good for a flat amount off the Item Total equal to the amount they'd spent, with a Max Use Count of 1 (any unspent discount amount would be forfeit and could not be retained to spend on a future order).
