May a seller offer a reward on a sale, which he explicitly limits only to cash sales? Is the buyer’s forfeiting the receipt of the reward in exchange for delayed payment considered ribbis?
On one hand your question seems to be similar to “psika” because you are offering the customer two differant prices. However when thinking about it, it seems to be permitted, and very differant then offering one price for cash and another for credit. The reason is because when someone demands a higher price for credit, when he will receive the money at a later date, he is taking extra oney because the payment is being given to him later. Therefore when the borrower (the customer) pays extra, it appears to be ribbis. In our case there is no loan here, just a regular sale, with a come on of a gift given from the seller to the buyer. If it would end up being a loan, i.e. (the money would be paid later), never the less, the seller is still the lender, and he is giving to the buyer, so it is the opposite of ribbis and permitted.
Even though paying cash comes out advantageous, paying via credit is not ribbis because the price essentially is the same whether it is cash or credit. The present that is being given is not part of the actual price of the item, rather a come-on, to encourage people to pay right away, therefore it would not be mefaresh.