It is not permissible to obtain any material gain or benefit in return for a loan (qard). The Prophet (peace and blessings be upon him) said: “Any benefit derived from a loan is riba (interest).” (Musnad al-Harith, 1/500; Sunan al-Bayhaqi: 5/350)
The amount given as a loan must be repaid exactly as it was, neither more nor less. In addition, no material benefit or advantage may be derived from the loan, such as using a vehicle for a period of time, staying in a house temporarily, or receiving gifts in return.
In Islam, a loan is given purely for the sake of seeking Allah’s pleasure; its reward is spiritual rather than worldly. If financial profit is intended, it should not be structured as a loan but rather as a partnership in which profit and loss are shared.
It should also be noted that in times of high inflation, in long-term loans, the loss in the value of money should be considered in order to avoid harm to the lender. This may be addressed by linking the loan to a stable value such as gold or, in some cases, the US dollar. However, this is a separate issue from earning a fixed return on a loan, and the two should not be confused.
In summary, earning a fixed income from lending money is not permissible.