This question pertains to math graduate programs in the US. Suppose a school sent a financial offer (TAship) to a student A and put a student B on the waitlist in the event that A declines the offer. Student A then declined the offer by signing the letter, so the school sent the offer to B.
My question: is it possible that the school will rescind the offer from B if A changed his/her mind later on and decided the take the offer given that
i) B has not yet accepted the offer.
ii) B has already accepted the offer.
iii) The date that A signed the letter to decline was before April 15th.