35 Bava Metzia 6/4-7

(4) [If] one hires a cow to plow on the mountain and he plows in the valley: if the plow breaks, he is exempt. In the valley and he plows on the mountain: if the plow breaks, he is liable. To thresh beans and he threshed grain, he is exempt. To thresh grain and he threshed beans, he is liable, because beans are slippery.

(5) The one who hires a donkey to bring wheat and he brought barley on it, is liable. Grain, and he brought straw on it, he is liable, since the bulk is difficult to carry. A letekh of wheat, and it carried a letekh of barley, he is exempt. But if he increased the load, he is liable. How much must he increase the weight in order to become liable? Sumkhos says in the name of Rabbi Meir: One se'ah for a camel, and three kabin for a donkey.

(6) All craftsmen are considered paid guardians [concerning objects others have given them to work on]. But any that have said, “Take what is yours and bring the money,” are considered as unpaid guardians. “Guard for me and I will guard for you" - he is a paid guardian. “Guard for me,” and the other said, “Place it before me” - he is an unpaid guardian.

(7) If he loaned him with a pledge, he is a paid guardian [of the pledge]. Rabbi Yehudah says: If he loaned him money, he is an unpaid guardian, [but] if he loaned him produce, he is a paid guardian. Abba Shaul says: It is permitted for a person to rent out the pledge of a poor person in order to thereby reduce the debt, for he is like one who returns lost property.