What should one do if he borrowed money from a friend in another country, then fell into difficult circumstances and was unable to repay the debt, and lost contact with his creditor friend, yet he strongly desires to repay his debt?

The debtor who is able to work and earn must repay his debt, as stated in Al-Shaybani's book, Kitab al-Kasb (The Book of Earning): "If he owes a debt, then earning what he can repay his debt with is an obligation upon him, because repaying the debt is a direct obligation upon him." And whoever sincerely intends to repay, Allah will help him, for the Prophet, peace and blessings be upon him, said: "Whoever takes people's money intending to repay it, Allah will repay on his behalf, and whoever takes it intending to destroy it, Allah will destroy him." If you exert your utmost effort to reach the lender and cannot find him, then give the money in charity on his behalf or spend it on acts of righteousness and the general public interests of Muslims, as Ibn Taymiyyah mentioned that money whose owner cannot be identified should be spent on the interests of Muslims.