Bechorot 31 - Selling the Meat of a Firstborn Animal
Any male firstborn animal must be given to a Kohen, but if it has a blemish, the Kohen can sell it to others. Since the firstborn was sanctified at birth, then even after it received the blemish, it must be treated with respect. In fact, this is true about any sacrifice if it got a blemish and was redeemed, but there are some differences between firstborn and sacrificial animals.
The redemption money of a blemished sacrifice goes to the Temple, where it is used to bring another sacrifice. Therefore the Sages, following the rule that the Temple is always treated preferentially, tried to maximize the Temple's profit. The meat of such sacrifices can be sold in a market, and it can be weighed in a regular way. However, the meat of a firstborn animal has to be sold by the Kohen from his house and cannot be doled out using weights. Instead, a piece of regular meat can be weighed, and the meat of the firstborn is then weighed against this meat. People will pay less in a limited market and without precise weight, but that is acceptable.
Art: William Hunt - Early Morning in the Village Cattle Market