[On Selling an Undivided Share (mushāʿ ghayr maqsūm)]

Answer (and Allah grants success and guidance):
Selling an undivided share in quarries that are later apportioned by tax liability (al-gha rim) or by head-count (al-d ak r) is invalid, due to: the prohibition of selling the khums until it is seized, the prohibition of selling what has not been received, and the prohibition of selling alms before it is received.
Quarries apportioned by gha rim or d ak r by mutual consent are public rights, just as khums is a public right. Disposition by sale and the like is not valid until the right is divided among its people and each takes possession of his portion; once divided, sale and the like are valid.
There are distinctions between public rights, private rights, and ownership:
1. By public rights we mean: the grazing of a tribe (or two), or the people of a village (or two); their commons (sāḥa) and their woodlots.
2. By private rights we mean feeder streams whose water flows to specific orchards, each orchard having an assigned measure of that water.
Private right: Its holder is entitled to it in consequence of ownership, and this right is not extinguished by death; rather, it passes by inheritance along with the property.
Public right: Each person among the people who share a public right (such as grazing or firewood) is entitled to his share so long as he is alive. When he dies, his right ends and does not pass by inheritance; it remains with the surviving members of the village or tribe.
So, if all those entitled to a public right mutually agree to divide it among themselves—by al-ghārim (tax-liability assessment) or by al-dhakīr (head-count)—and they do in fact divide it, and each takes possession of his portion, then it becomes a private right. Before that, no individual among them has any specific private right.
Accordingly, whoever sells a private right before it exists and is obtained—merely on the hope that it will come to be in the future, whether sooner or later—his sale is void, because one of the pillars of sale is missing.
However, since such sales have become widespread, declaring them void would undoubtedly open the door to widespread corruption across the land. A duty is set aside when doing it would lead to a greater harm than the benefit of performing it. Thus a practical resolution is needed.
The remedy is either to strive for conciliation between the disputants—and if reconciliation succeeds, well and good—or else to oblige the seller to give the buyer plots (ḥabala) in proportion to the money actually received, and no more. If the seller has received the entire price, he must deliver the whole thing sold; if he has received nothing, then nothing is due.
We say this by way of reasoned judgment when a formal ruling is blocked, for several reasons:
1. We treat this under the rubric of undertakings. The buyer gave the seller a sum of money; the seller undertook to deliver such-and-such number of plots. If the buyer fulfills his undertaking as agreed, the seller must fulfill his in kind; but if the buyer fails in his undertaking, the seller is not bound to fulfill his. Proof is His saying, Exalted is He,: “So long as they are true to you, be true to them” [al-Tawbah:7].
2. Ordinary people have engaged in such sales extensively and believe them valid. The juristic maxim says: “We carry Muslims’ transactions on validity wherever possible.”
3. The seller’s statement, “Give me such-and-such amount of money and I will, after a time, give you such-and-such number of plots,” is a promise from the seller to the buyer. If the buyer gives the requested money, the seller must keep his promise, due to the stern warning against breaking promises—and this promise is further reinforced by its resemblance to a sale.
Objection: Allah said, “Judge between them by what Allah has sent down” [al-Māʾidah:49], and “Judge between people with truth” [Ṣād:26]. Your proposed resolution departs from that.
Response: A formal ruling of invalidity has become infeasible because of the overriding harms it would cause, and the Lawgiver has permitted leaving some obligations in cases of necessity—“If you fear [danger], then [pray] on foot or mounted” [al-Baqarah:239].
If judging strictly between the litigants would lead to bloodshed and mischief on earth, it is permissible to leave it. The Messenger of Allah (May Allah bless him and his family and grant them peace) left off applying the ḥadd in the slander of ʿAbd Allāh b. Ubayy out of public interest.
Where necessity permits leaving the strict rule, the judge should still resolve the dispute and cut off strife in the way closest to right and justice—and what we proposed above is closer to the right, as we have explained.
Second issue:
If someone sells an undivided share, then the common property is partitioned and the seller’s portion turns out to be less than he sold, the buyer is entitled only to the seller’s actual portion; he has no claim to compensation from elsewhere, because the sale related to a specific undivided mass.
Third issue:
If someone sells an undivided share and dies before partition, his right in the quarry ceases. If later the quarry is divided by al-ghārim or al-dhakīr, the buyer gets nothing from it, because it turns out the seller ultimately had no share there. The buyer then has recourse against the heirs for the money he paid.
Fourth issue:
If the quarry is divided by head-count or tax-liability and the seller’s portion is less than what he sold, he is not obliged to make up the buyer’s shortfall from the portions of his children, even if he is their liable party or guardian.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.2