Mishpatim

The Bechorim only lost their rights and required redemption after the sin of the golden calf, when so many firstborns sinned. Afterwards they were redeemed onto the Leviim and the extra ones paid money. If so how could the Jews be commanded to redeem their firstborns in Shemos 22,28, if according to most Rishonim that happened before the sin of the golden calf?