[Two partners in a well: one requests digging it deeper, the other refuses]

Answer—and Allah is the One who grants success: According to the madhhab—as in the marginalia of Sharḥ al-Azhar—if one of the two partners requests digging the well to increase its water, his request need not be answered—unless it is known that, if it is not dug, its water will diminish. End (Established). (Í)
I say: If there was a stipulation between the two partners—agreed upon before digging—that, for example, if the well needs deepening or the like, it is upon both partners, then the partner must fulfill it, and the judge rules accordingly; for Allah Most High said, “O you who believe, fulfill all contracts.” [al-Māʾidah:1] And in the report: “Muslims are bound by their conditions.”
But if he requests deepening and there had been no prior agreement by the two partners to that effect at the time of need, then—so far as appears to me—there is no basis to compel the one who refuses to deepen it, unless there is a custom current among partners in wells; in that case it is binding, because if custom has run upon such a matter, it takes the place of a stipulation. Otherwise, there is no obligation upon the one who refuses.
The proof for that is the ḥadīth: “The property of a Muslim is not lawful (to take) except with the cheerful consent of his soul.” However, if the partner wishes to deepen a well that needs deepening due to scarcity of water, then he may deepen it, and his partner may not prevent him. Thereafter, if the one who refused deepening wishes to take his share of the well’s water, let him take from the water what he had been taking before the deepening; so if he had been irrigating one hundred rope-lengths in his allotted days, let him take what irrigates that amount—even if in some of (the other’s) days. Or the increase due to deepening may be assessed and allotted to the digger. If, for example, the water increases by a third, that additional third is exclusively for the digger, and the remainder is divided between the two partners. If the refuser later wishes to share with his partner—the digger—in the water as it was originally, then let him share with him in the expense (loss) as well.
What we have stated is nearer to conformity with foundational principles, such as His saying, “It will have the consequence of what it has earned, and it will bear the consequence of what it has committed.” [al-Baqarah:286]; and His saying, “And you will not wrong others , nor will you be wronged.” [al-Baqarah:279]; and His saying, “Do not consume one another’s wealth unjustly—except it be a trade by mutual consent among you.” [al-Nisāʾ:29]; and (the report), “The property of a Muslim is not lawful (to take) except with the cheerful consent of his soul.”
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2