Reference for Bekhorot 68:7

כי קניס ר' אליעזר בממונו בגופו לא קניס

As regards his property [i.e.the firstling], one can say that he may do it [in either case]<span class="x" onmousemove="('comment',' For we argue that if by causing the blemish he is permitted to slaughter the animal, then he benefits thereby, and if he has to wait till another blemish appears, then he has lost nothing, as in any case he intended waiting for another blemish to appear. We therefore condemn him never to slaughter the firstling, so as to prevent him causing blemishes.');">⁴</span>

Jastrow