Midrash for Bekhorot 36:18
מהו דתימא
Have we not learnt: For R'Jose used to say that wherever the priest has [a beast] in its stead it is exempt, whereas R'Meir makes him liable?<span class="x" onmousemove="('comment',' Now, if the reason of R. Meir with reference to the firstling is because the priest can make his claim on two grounds and therefore R. lose argues for exemption, maintaining that the priest cannot say that if it is a firstling then it belongs entirely to him, since he holds that it is as if the priest had, after acquiring the firstling, sold it to the Israelite. But if you maintain that the reason of R. Meir is lest the law of the priest's gifts be forgotten, why does R. Jose give the reason that the priest has a beast in its stead, since possibly R. Meir himself might have exempted him on that ground. (Rashi) .');"><sup>6</sup></span>