What should a person do who bought land to build a mosque with an interest-based loan due to the unavailability of a mosque in his area, believing that there are no interest-based loans in Saudi Arabia?

It is not permissible to borrow with interest (riba) whatsoever, even if the loan is for a good deed, such as building a mosque, because Allah is good and accepts only what is good, and usury is forbidden in Islamic law. As for one who borrowed with usury while ignorant of its prohibition, there is no sin upon him for what has passed, but he is obligated to avoid it in the future.