Reference for Bekhorot 35:37

יחלוקו ר' עקיבא אומר

Said R'Eleazar: All [the authorities concerned] agree that an animal which is a doubtful first-born, since the priest has [a beast] in its stead,<span class="x" onmousemove="('comment',' Where for example a female and a male are born and there is a doubt as to the first-birth, since the priest received nothing in its place, the animal grazes until it is blemished and is therefore liable for the priest's gifts, for in such a case you cannot argue that it is as if it had been acquired by the priest and subsequently sold to the Israelite, as the priest received nothing in return.');">¹³</span>

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