Reference for Bekhorot 36:13

מהו דתימא

For R'Meir only makes him liable for the priest's gifts in connection with an animal which is a doubtful first-born, since the priest can make claim upon him from two sides,<span class="x" onmousemove="('comment',' If it is a firstling, then it is entirely his, and if not, then it is hullin and subject to the priest's gifts.');">³</span>

Jastrow