Are early-bird discounts ribbis? For example, "buy before March 1, 10% off", or "tickets $12 in advance, $15 at the door" Inspired by http://myparnasa.com/collecting-interest-ribbis/
I will ask the question using the following scenario: Reuvan wrote a sefer that he was charging for it 50 Shekels. Came along Shimon who wanted to buy the sefer. At that time he didn't have money on ...