הלכה על בבא מציעא 7:11

Accordingly, “batter” should be excused from payment since the bat was broken as a result of normal usage. Moreover, the stipulation for payment should be invalid since it contradicts Torah law. Indeed, the Mishnah (Bava Metzia 7:11) states “kol hamatneh al mah shekatuv baTorah tena’o batel”, a stipulation that runs counter to Halachah is invalid.