Baba Metzia 4:1-2

Baba Metzia 4:1

Gold can be used to acquire silver (wherever it may be) but silver cannot be used to acquire gold (without actually taking possession of it). Copper can be used to acquire silver but silver cannot be used to acquire copper. Bad coins (worn out or no longer negotiable) can be used to acquire good ones but good ones cannot acquire bad ones. An unstamped coin can be used to acquire a stamped coin but a stamped coin cannot acquire an unstamped coin. Movable property can acquire stamped coins but stamped coins cannot acquire movable property. (The issue in all of these cases is what’s considered currency and what’s considered merchandise. Currency alone cannot be used to acquire merchandise without also taking possession of it.) The general principle is that any movable property can be used to acquire other movable property.

Baba Metzia 4:2

If a person took produce from the seller but did not yet give him any money, he cannot cancel the transaction (because he already took possession). If the buyer paid the seller but did not yet take the produce from him, he may cancel the transaction (because money alone is not an acquisition). Nevertheless, the Sages said that God, Who punished the generation of the flood and the generation of the tower of babel, will also punish whoever does not keep his word. Rabbi Shimon disagrees, saying that whichever party is in possession of the money has the legal advantage (i.e., that person can still void the transaction).