Charging less money for immediate payments on a product:
When doing a sale, it is Rabbinically forbidden for the Jewish seller to offer a Jewish buyer a payment option in which immediate payment is less than the amount of a delayed payment. This applies even if the delayed payment option is the true market price of the product while the immediate payment option is a decrease of the products market price. For example, a seller may not tell the buyer during the negotiations of the sale “If you pay me now, I will give it to you for ten dollars, while if you pay me later I will give it to you for 12 dollars.” Likewise he may not say “If you pay me now, I will give it to you for less than the sale price, while if you pay me later you will have to pay the regular sale price.” This prohibition applies even if the seller does not explicitly mention this offer to the buyer, but simply tells him a price for delayed payment that is higher than the regular market price. [This prohibition applies whether one is offered to pay the entire payment at a later date for a greater price, or only part of the payment at a later date for a greater price.]
After the sale: It is Biblically forbidden to give the above offer after the sale already took place, as doing so is considered Biblical interest. For example, if the buyer and seller agreed on an immediate payment price and the buyer acquired the product, the seller may not tell the buyer “I’ll make you a new offer. If you pay me now, I will give it to you for ten dollars, while if you pay me later I will give it to you for 12 dollars.”
If the seller transgressed and made the above offer to the buyer, may the buyer nevertheless purchase the product?
It is forbidden for the buyer to purchase the product for the later payment option, as buy doing so he is performing Ribis, which is forbidden on both the lender and borrower. However, he may still take the immediate payment option, as he is not actually giving any interest to the seller, as he paid the entire lower price immediately. This applies even if the immediate payment option is less than the market value, so long as he acquires the product at the time of the sale.
May one charge more money for a non-cash payment such as a check, bank transfer or credit card payment and the like?
Yes. One may charge more money for non-cash payments if the additional price is to compensate for the work and fees that the seller must go through in order to receive the payment. Thus, for example, if the one’s bank charges a fee for the check, or bank transfer, one may tell the buyer that he can either pay the regular price in a cash payment or pay through check or bank transfer with the additional bank fee which will be charged. It is however forbidden for the seller to say that he is charging him more due to delayed payment, even if in truth the seller has extra fees to pay due to this delay. The amount that the seller may charge extra for the non-cash payment must be commensurate to the fees he will entail, and it is to be explicitly mentioned to the buyer.
 Admur Ribis 18; Michaber 173/1; Mishneh Baba Metzia 65a
 The reason: This is forbidden due to Avak Ribis as the moment that the buyer acquires the item he becomes liable to pay and in essence now owes the money to the seller. This applies irrelevant of the price of the object, whether it is expensive or cheap, so long as no stipulation was made on the acquisition of the sale. Accordingly, if the seller asks to receive a larger payment when being paid at a later date as compensation for allowing this delay, it appears like he is taking interest on the delay of payment, of the debt which was incurred at the time of the sale. This is in contrast to renting an item, in which the payment is only liable at the conclusion of the rental. [Admur ibid]
 Admur ibid; implication of Gemara ibid; Ramban Milchamos 36b
 Admur ibid
 Admur ibid and 4
 See Mishpat Ribis 20/1
 Beis Efraim Y.D. 41; Mayim Rabim 38; Mishpat Ribis 20/8; Bris Yehuda 22 footnote 6