John and Mary are brother and sister.Upon the death of their mother, it was discovered that the mother's will left the bulk of her property to Mary.John threatened to contest the will.Mary wrote to John stating, "If you tell the truth in court, I will give you $10,000."John told the truth in court and now demands $10,000 from Mary.Must she pay?Decision?

Mary wins because John was stupid enough to believe her and thus didn't contest the will.

John wins because he performed his end of the bargain by telling the truth in court and so Mary owes him the money as promised.

Mary wins because the agreement with John lacked consideration.

John wins because it is only fair that he should receive his fair share of his mother's property instead of Mary getting it all.