Bava Metzia 44 - Last Thoughts of Appropriation - and on to Sales
If a custodian intended to misappropriate a deposit, making a declaration in front of two witnesses, from that moment he is liable for any mishap that might befall it - so say Beit Shammai, but Beit Hillel say that he is not liable until he acts, because of "... if he did not lay his hand upon the goods ..."
By Rabbinical law, when someone pays money for a movable object, he does not acquire it yet, and either party may renege. To finalize the sale, the buyer must take possession, by lifting the object or drawing it to himself. When gold is exchanged for silver, gold is considered merchandise whose transfer completes the sale.