פירוש על קידושין 55:10

The Talmud asks what this item is that is being exchanged for another item. If that item is money, then barter can be performed with money. This would mean that if I give you money for your cow, then I have acquired your cow and neither of us can renege. The problem with this is that we have learned that money cannot be used to acquire objects.

Rav Judah explains the mishnah—for an item to be used as barter it must be something that does not have a value that can be expressed in its own terms. For instance, I could say a Hershey Bar is worth four Hershey kisses or two Goldenberg’s chews (I really loved those). But I cannot say how much a Hershey bar is worth in terms of Hershey bars. But a coin’s value is expressed on its own terms—a dollar is worth a dollar. Therefore, coins cannot be used in barter.