[The Ruling on Stud Fees (ʿUsb al-Faḥl)]

The answer—and Allah is the granter of success and aid—is that what appears to me is permissibility, on the condition that he cannot find a bull except for a fee. This is because the prohibition is directed to the owner of the sire—that he not take a price for his sire’s semen. If the cow’s owner is in necessity, it is permissible for him to pay the fee, while it is prohibited for the sire’s owner to take it and consume it. If the cow’s owner can find a bull without payment, it is not permissible for him to turn to one who takes a price—not because he is forbidden to pay, but because doing so would aid the sire’s owner and affirm him upon the wrongdoing.
If it is said: Aiding the sire’s owner and affirming him upon wrongdoing is impermissible in both cases; what is the evidence for permitting it in one case and not the other?
The answer: The state of concession (rukhs ah) differs from the state of strict resolve (ʿazīmah). The state of permissibility here is the state of necessity, and the ruling under necessity differs from the ruling by choice.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2