Commentary for Kiddushin 56:6
מילתא דשכיח גזרו בה רבנן מילתא דלא שכיח לא גזרו בה רבנן
HOW SO? IF ONE BARTERS THE MONEY OF AN OX FOR A COW, OR THE MONEY OF AN ASS FOR AN OX.<span class="x" onmousemove="('comment',' E.g., A sells an ox to B for a certain sum of money, and B takes possession, thereby becoming indebted to A for the purchase price. Then B says: 'I will give you a cow for the purchase price of the ox,' to which A agrees. Now, though this is theoretically a fresh transaction, viz., B sells a cow to A, the money owing by B for the ox being regarded as though delivered to him by A for the cow, and it is a principle that the delivery of money alone does not consummate a purchase, it does so here, and neither can retract, i.e.,it is barter, not payment.');"><sup>2</sup></span>