Bava Metzia 78B

נקרא גזלן.

is called a robber. Since he contravened their agreement and did not receive approval of the other party, it is considered as though he stole the animal from its owner. A robber is liable for any damage that occurs, even due to circumstances beyond his control.

הי רבי מאיר? אילימא רבי מאיר דצבע, דתנן: הנותן צמר לצבע לצבוע לו אדום וצבעו שחור, שחור וצבעו אדום, רבי מאיר אומר: נותן לו דמי צמרו.

The Talmud asks: Which of R' Meir’s statements indicates that this is his opinion? If we say that it is R' Meir’s statement with regard to a dyer, this claim can be refuted. As we learned in a Mishnah (Bava Kamma 100b): With regard to one who gives wool to a dyer to dye it red for him, and instead he dyed it black, or to dye it black and he dyed it red, R' Meir says: The dyer gives the owner of the wool the value of his wool.

רבי יהודה אומר: אם השבח יתר על היציאה ⁠– נותן לו את היציאה, ואם היציאה יתירה על השבח ⁠– נותן לו את השבח.

R' Yehuda says: If the value of the enhancement resulting from the dying exceeds the dyer’s expenses, then the owner of the wool gives the dyer the expenses. And if the expenses exceed the enhancement, he gives him the value of the enhancement. This apparently indicates that because the dyer diverged from the agreement, R' Meir considers him a robber, which is why the dyer must refund the entire value of the wool.

ממאי: דלמא שאני התם דקניא בשינוי מעשה?

The Talmud challenges this proof. From where do you know that this Mishnah is the source of R' Meir’s opinion with regard to reneging on an agreement made with an employer? Perhaps it is different there, as the dyer acquired the wool by means of the physical change in a substance, and it is for that reason, not due to his divergence from the agreement, that he is comparable to a robber.