Reference for Bava Kamma 229:8
אימא אף דין עם הטבח מהו דתימא אין מתנות כהונה נגזלות קמ"ל
and there is nevertheless here a difference of opinion? For we learnt: If one asked another to sell him the inside of a cow in which there were included priestly portions he would have to give it to the priest without deducting anything from the [purchase] money; but if he bought it from him by weight he would have to give the portions to the priests and deduct their value from the [purchase] money.<span class="x" onmousemove="('comment',' Hul. X, 3. ');"><sup>13</sup></span> And Rab thereupon said that the [last] ruling could not be explained except where it was the purchaser who weighed it for himself, for if the butcher<span class="x" onmousemove="('comment',' I.e., the vendor. ');"><sup>14</sup></span>