Bava Metzia 80: Rental Agreements Gone Wrong

3 mishnayot! 1 - If one rents a cow to plow a mountain, but plows in a valley... how does that difference (and the reverse) make a difference in terms of liability? 2 - Renting a donkey to transport wheat, but barley is transported instead - and how either weight or volume may make a difference in terms of the renter's liability. 3 - Terms of renting vs. a paid guardian, plus terms of helping a poor person in this regard.