הלכה על בכורות 62:35

From the laws of the commandment is that which they, may their memory be blessed, said (Zevachim 56b) the tithe of beasts was eaten completely by the owners in Jerusalem, and the priests have nothing in it. However it was slaughtered in the courtyard and they would offer its entrails and sprinkle its blood with one sprinkling across from the base [of the altar]. And if it developed a blemish, it is eaten in any place. But the Sages forbade to sell it nonetheless, and even if it is a blemished one. And even if it is slaughtered is it forbidden to sell it [as a] decree lest one will sell it alive (Bekhorot 31b). And therefore they said that we do not weigh one piece from it against [another] piece - as it appears like selling. And [this is the law] even though this thing is permitted to do even with an unblemished first-born; as they, may their memory be blessed, said (Bekhorot 31a) [that] the priests that are designated for a first-born are permitted to weigh one piece against [another] piece. And that which they, may their memory be blessed, said [regarding] the tithe of beasts that is slaughtered, [that] it is permissible to sell its fat, its tendons, its skin and its bones - as the they only forbade to sell its meat alone. And if the bones were expensive and he included the value of the meat in [that of] the bones, it is permitted. And Rambam, may his memory be blessed, wrote (Mishneh Torah, Laws of Firstlings 6:5) that it appears to him that [in the case of] one who sells the tithe, the purchaser does not acquire [it]. And therefore the seller is not lashed for it - since his actions were not effective. And [it is] like one who sells the dedications of the priests, such that the buyer did not acquire [it], and like one who sells a woman (captive) of beautiful form, such that the buyer did not acquire [it] - as is elucidated in its place. To here [are his words]. And the rest of the details of the commandment are elucidated in Tractate Bekhorot and in Tractate Maaser Sheni at the beginning.