Is it permissible to donate money that is of a questionable nature—estimated at 20% of the monthly income—to pay off my father's usurious debt?

If someone possesses money tainted with an element of haraam (unlawful) and wishes to purify it, they must relinquish the questionable amount if they know it. If they are unaware of the exact amount, they must exercise caution until their liability is cleared. As for the usurious (riba-based) interest accrued on the father's debt to a riba-based bank, he is not obligated to pay it; rather, he is only required to repay the principal amount of the loan. However, if the father is compelled to pay the usurious interest to avoid penalty and has no money, it is permissible for the questioner to pay this interest on his behalf from the suspicious money. It is also permissible for him to pay the principal debt on his behalf if the father is unable to fulfill it.