Is borrowing from a bank to buy a car and expand a business, considering the bank a partner with a share of the profits to ensure seriousness, permissible or forbidden?

We understand that you have doubts about the prohibition of interest-based loans and wish for their permissibility by analogy with Mudarabah (profit-sharing partnership). The form of Mudarabah you described is not permissible if the capital provider stipulates a specific profit to be paid by the working partner, as it ultimately leads to an interest-based loan.

Mudarabah is only valid if the working partner is not liable for the principal capital and the profit is not fixed at a specific amount, but rather as a percentage for each party. Loss, in this case, is borne by the capital and not by the working partner. This differs from a guaranteed loan with usurious interest that increases with delayed repayment.

If the bank desires a permissible profit, it should sell a commodity it owns to the client for a known price, even if that price is higher than its market value, in accordance with the Murabahah (cost-plus financing) sale and installment system. This is what Islamic banks mostly do. Allah says: "And Allah has permitted trade and forbidden interest" [Al-Baqarah: 275], and He says: "O you who have believed, do not consume one another's wealth unjustly but only [in lawful] trade by mutual consent." [An-Nisa: 29].

Profit in trade is permissible, while in a loan it is forbidden. The disbelievers said: "Trade is like riba," so Allah revealed: "Those who consume interest cannot stand [on the Day of Resurrection] except as one stands who is being beaten by Satan into insanity. That is because they say, "Trade is [just] like interest." But Allah has permitted trade and forbidden interest. So whoever has received an admonition from his Lord and desists may have what is past, and his affair rests with Allah. But whoever returns [to dealing in interest] – those are the companions of the Fire; they will abide eternally therein." [Al-Baqarah: 275].

The basis of a loan in Islamic law is assistance and benevolence, and taking compensation for it negates this objective. "Every loan that brings a benefit is usury." To obtain a car or money, follow legitimate paths such as Murabahah or Tawarruq transactions with an Islamic bank, whereby the bank buys what you want and then sells it to you on installment, and then you sell it to a third party.