Related for Beitzah 54:8

What is "One shouldn't reserve one's cattle for a part of a piece of cattle only on a holiday"? He shall not say to anyone: I am your partner for one sela, or: I am your partner for two sela, or: I am your partner for three sela. Rather he says unto him: I am your partner for a half, a third or a quarter of the beast. but the otherwise say: The scale must not be considered at all; rather he weighs it in his hand and lays it down. If the butcher is skillful, let him not weigh, because his hand is like a weight; but he may cut it off with an instrument and give the flesh to one or another.