If the seller gives the option of paying in cash for a small discount as opposed to paying in card or check, is this considered Ribbis?
Sources are appreciated.
There is no problem in this.
Although there can be a problem if a discount is offered for immediate payment rather than credit payment, in the case of the question this does not seem to be the issue, but rather a question of convenience (avoiding the credit card charge and the matter of cashing in the check; I trust that the issue is not avoiding payment of tax).
Even if the issue is payment up front vs. intallments, it is permitted to give a discount if the item is in stock. This is by contrast with the case of paying extra for installments.
See Shulchan Aruch, Yoreh De’ah 173:7; the Rema adds that the seller is believed to say that he has the item in stock.