Does a Muslim sin if he follows the fatwa of the Dar al-Ifta allowing a loan to buy an apartment, while another source deems it forbidden, in a case where he is in need of the loan?

Borrowing with usury is a grave sin that should not be undertaken except in cases of extreme necessity, such as fear of perishing. Purchasing an apartment for housing is not a necessity. An ordinary person should ask someone whose knowledge and piety they trust, and act upon what their heart feels at ease with. It is not permissible for them to act upon a mufti's fatwa if their heart is not at ease with it and they feel unease about it, based on the hadith: "Consult your heart, even if people give you a fatwa and they give you a fatwa." The mufti's fatwa does not justify for the questioner what they know to be inherently false, or what they doubt, or are ignorant of, or if there is doubt about the mufti himself, or his favoritism, or his non-adherence to the Qur'an and Sunnah. If the lack of trust is due to him, they should ask others until reassurance is achieved.