Is it permissible for the borrower to agree with the lender to repay the loan amount, in addition to a percentage of the profit from selling the land purchased with the borrowed money, or to set a price for it so that the borrower can repay the lender his share of the profit, in the event that selling is not possible?

It is forbidden to return the borrowed amount with an increase in exchange for delaying repayment; this is from the usury of Jahiliyyah (pre-Islamic era), and every loan that brings a benefit is usury. Your friend is only entitled to the amount he lent you. You must promptly repay the debt if you are able. If you are unable, then he must grant you a respite until your circumstances ease, and it is not permissible for him to take an increase in exchange for this respite, as Allah Almighty says: "And if there is one in [financial] hardship, then [let there be] postponement until [a time of] ease."