Is it obligatory to return money borrowed from a deceased person whose relatives are unknown, at the same value as at the time of borrowing? And is it permissible to give it as an ongoing charity?

As long as the amount your father took was a loan, it is a debt owed by him that must be repaid in kind, not by its value. If the lender cannot be reached and there is no hope of doing so, the amount should be given as charity for the benefit of Muslims. If the lender reappears, he is given the choice between approving the charity or reclaiming his money. This debt must be paid from the deceased's estate before its distribution, because the soul of a believer remains suspended by his debt until it is paid off.