[Judgments that upheld sales of rights]

Answer: The sale of rights is a matter of ijtihād; thus, one does not denounce a judge who ruled for its validity.
Moreover, judges may have considered custom among people regarding quarries and wild growth, noting that litigants willingly sought judgment according to that ʿurf, content with rulings issued on that basis. By submitting to the judge, they effectively ask to be judged by the prevailing custom;
the judge is thus like an agent for the disputants to resolve their matter by what they agreed to, not otherwise. The parties’ consent to be bound by custom gives the judge warrant to rule accordingly.
If it is said: Allah, Exalted is He, says, “Whoever does not judge by what Allah has sent down—they are disbelievers” [al-Māʾida:44].
We say: in such cases, judgment is by what Allah sent down, because the parties’ consent and commitment to the terms of ʿurf are a legal pathway for adjudication and binding orders: one litigant says, “If my opponent’s document is valid, I accept to hand over the disputed matter,” or “Let my opponent swear his deed is valid, and I will concede and relinquish my claim,” and the other opponent says like that, and the judge assesses the validity and priority of documents and when to require an oath.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2