Mishnayos Chullin Perek 5 Mishnah 4

On those four occasions, one compels the butcher to slaughter animals even against his will; even if there is a bull worth one thousand dinars and the buyer has only one dinar worth of meat, i.e., he already paid the butcher for one dinar’s worth of meat, one compels him to slaughter the animal and give him a dinar’s worth of meat. Therefore, if the bull dies before slaughter, although no act of acquisition was performed, it dies at the expense of the buyer, and he loses his dinar. But during the rest of the days of the year it is not so. On other days, until the buyer performs the act of pulling to assume ownership of the portion of the bull that he is purchasing, the bull remains in the butcher’s possession. Therefore, if the bull dies before the transaction is complete, it dies at the expense of the seller, who returns the buyer’s money.