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/


Question: Why do we use singular verb with third person after gerund?
Answer: Because the world and the paint aren't the subjects of the sentences, which means there's not a fixed point in time when the world goes or the paint dries. Consider this sentence:


  The clock ticks


The clock is the subject, so we conjugate "to tick" into the present simple. However if the clock is not the subject, then the verb "to tick" doesn't get conjugated:


  Electricity makes the clock tick


This is because the ticking doesn't just happen in the present tense, the electricity gives the clock the ability to tick, which is why we use the infinitive.