[Increasing the Price of a Commodity When You Know the Buyer Wants It on Credit]

The answer—and Allah is the granter of success: In the report narrated by Imām Zayd ibn ʿAlī (Peace be Upon them), there is a prohibition of “two conditions in a sale.” Imām Zayd (Peace be Upon Him) explained this with examples such as: a man says regarding a commodity, “If it is for cash, then [the price is] such-and-such; if on credit, then such-and-such,” or, “To such-and-such a term, [the price is] such-and-such; to such-and-such a term, [the price is] such-and-such.”
Yes, what outwardly appears regarding the sale that occurred in the question is validity and permissibility, because those two stated conditions were not mentioned therein—that is what appears.
However, the scholars have said that an implicit condition is like an explicit one. So if the two parties bargaining over the commodity mutually agreed to a price higher than the customary market price—equal to it or to half of it—on account of the credit, while both know the customary price with this seller and with others, but they did not explicitly mention the condition at the time of contract or within the contract, then in reality the sale falls under the transactions that are prohibited.
The clarification of this: Allah, Exalted is He, only forbade ribā and what is attached to it among the transactions that have been prohibited because of the injustice in them, which is consuming people’s wealth wrongfully, as indicated by His saying: “But if you repent, you may have your principal—you do no wrong, nor are you wronged.” [Al-Baqarah:279].
The increase that the seller stipulates and adds to the price due to the term is a form of injustice and of consuming people’s wealth wrongfully. The proof for this is that Allah, Exalted is He, forbade ribā al-nasī’ah, which is an increase over the principal in exchange for time, and the sale mentioned is, in reality, of that kind.
The evidence that it is impermissible to exchange time for money, along with what we mentioned regarding ribā al-nasī’ah, is His saying: “And if someone is in hardship, then [let there be] postponement until [a time of] ease.” [Al-Baqarah:280]. If we recognize that Allah Most High decreed that because of the injustice and wrongful consumption of wealth involved, then the prohibition of the sale applies equally whether the two conditions are explicitly mentioned or implicitly intended.
On that basis, the sale mentioned in the question is prohibited due to the injustice in it, which is consuming wealth without right—namely, in exchange for time, and you have learned from what has preceded that increasing [the price] in exchange for time is prohibited.
Accordingly, it is forbidden for both seller and buyer to enter into such a sale. The buyer may be permitted to enter such a sale only if he is compelled to it by necessity—unlike the seller, for whom there is no necessity; thus, it is not permissible for him to enter into it at all. This is due to His saying, after mentioning certain prohibitions: “Excepting that to which you are compelled.” [Al-Anʿām:119], and His saying: “But whoever is compelled by severe hunger, with no inclination to sin—then indeed, Allah is Forgiving, Merciful.” [Al-Mā’idah:3]; and in the well-known report: “In cases of necessity, the prohibited becomes permissible.” And He, Most High, said: “He has not placed upon you in the religion any difficulty.” [Al-Ḥajj:78], and: “Allah intends for you ease and does not intend for you hardship.” [Al-Baqarah:185].
Yes, the matter is one of scholarly disagreement; however, precaution lies in what we have mentioned, to be certainly safe from falling under the threat directed at the consumer of ribā in the Book and Sunnah. And in the hadith: “Believers pause at doubtful matters,” and: “Leave what makes you doubt for what does not make you doubt.”
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1