One who wishes to trim the leaves of vegetables to lighten his burden may not discard [the leaves] until he tithes [them]. One who purchases vegetables in the market and decides to return [them] may not return [them] until he tithes [them], since [the transaction] is lacking nothing but counting. [If] one was standing and purchasing [a load of vegetables], and saw another load better than [the first], it is permitted to return [the first load], since he did not draw [it].