[Ruling on a Reward (Juʿālah) for Standing Surety for the Price of a Sale]

Answer—and Allah grants success: What appears to me—Allah knows best—is that there is no impediment1 to the surety taking the reward absolutely—whether from the seller or from the buyer, whether it was stipulated or not, by contract or without a contract.
The proof is that the default in transactions is validity and permissibility until evidence of prohibition appears—except in limited issues such as usurious matters and what is attached to them, taking wages for performing an obligation or a prohibited act, the wage of a songstress or a prostitute, the price of a dog, and so forth.
The matter asked about resembles none of those; thus we said: the wage or reward is lawful for the surety.
If it is said: What is the evidence that the default in transactions is validity and permissibility?
We say: The evidence is that Allah, Exalted is He, brought Islam while people were engaging in various types of transactions; He, Exalted is He, forbade some of what they practiced and approved the rest.
This is unlike acts of worship: prayer, for example, was only known by Allah’s defining it. The default in it is invalidity until it accords with the command of the All-Wise Lawgiver; it is not ruled valid until evidence of its validity is established—likewise fasting, ḥajj, zakāh, and the other acts of worship.
And from this it is that Allah, Exalted is He, said: "Or have they partners who have ordained for them a religion to which Allah has not consented?" [Ash-Shūrā:21], and in the hadith: “Pray as you have seen me pray,” and: “Take your rites from me.” And He, Exalted, said: "For every nation We have appointed religious rites which they are to observe." [Al-Hajj:67].
If it is said: The surety (kafīl) takes a wage or a reward (juʿālah) without a counter-value, so his consuming it is among what Allah has forbidden in His saying: "Do not consume one another’s wealth unjustly," [An-Nisa:29].
We say: The surety here has not taken a wage without a counter-value; rather, he has taken it in exchange for the guarantee and commitment, on account of which much toil and hardship may befall him, and in the end he may have to pay what he guaranteed. Hence there is the hadith reported from the Prophet (May Allah bless him and his family and grant them peace): “Revenue follows liability (al-kharāj bi’l-ḍamān .”
The meaning of the hadith—Allah knows best—is that if a buyer returns the sold item to the seller due to a defect that appears, for example, and he had benefited from some usufructs before returning it, those benefits are lawful for him and he is not obliged to return them, and that is in exchange for his having borne the liability for the sold item; for had it perished in his hand before he returned it to the seller, it would have perished from his wealth, not the seller’s. In this hadith is evidence that liability may be met with (i.e., warrants) money.
On the basis of what has preceded, the hire (ijārah) may be valid or invalid: it is valid if the wage is only for the bare guarantee and commitment without regard to pursuing and monitoring the one for whom the surety stands; and it is invalid if it is for the guarantee together with pursuing and monitoring the principal.
The reason for its invalidity in the latter case is the ignorance (indeterminacy) of the extent of that pursuit and monitoring.
If it is said: If the ijārah is invalid, then the surety may rescind; and if he rescinds, the suretyship is void—entailing obvious harm to the creditor, through procrastination over his money or its loss, or the loss of some of it—
We say: If the ijārah is invalid here, the guarantee does not become invalid. What is meant is that, if the ijārah is invalid and a dispute arises, the surety is not entitled to the wage named in the contract; rather, he is entitled in proportion to his work—ujrat al-mithl (the customary wage).
By our saying “the guarantee does not become invalid,” we mean: it is obligatory upon the surety to stand by and carry through with what he guaranteed, even if the ijārah is invalid; because:
1- Fulfilling promises is obligatory in the religion of Islam, and a Muslim is not excused for abandoning it except by a clear excuse. For this reason, the Messenger of Allah (May Allah bless him and his family and grant them peace) counted breaking promises among the signs of the hypocrites.
2- Because abandoning the suretyship may result in the debtor’s procrastination or the loss of the creditor’s money, or the loss of part of it.
Now, our judging the ijārah invalid in what we mentioned pertains to when the ijārah is as described, and that is according to what the jurists of the school indicate. Otherwise, what appears to me—Allah knows best—is that the aforementioned indeterminacy is pardoned when the intended purpose of pursuit and monitoring is known.
This applies to what cannot be precisely quantified at all—such as the work of the muṣaddiq (the collector of zakāh for the Imām), and such as the overseers of endowments, and the custodians of mosques and sanctuaries, and the like. The preferable view is that ijārah in their case is valid, not as some jurists of the school have said.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2