How should one deal with the sum of 55,0 EGP, which resulted from an interest-bearing investment certificate, after the mother's death and the cancellation of the certificate before its maturity, considering that the bank deducted 5,0 EGP from the principal and 10,0 EGP accumulated as usurious interest, and should one dispose of only 5,0 (the deduction from the principal) or 10,0 (the usurious interest) based on Allah's saying: "And if you repent, you will have your principal sums"?

It is forbidden to invest money in a usurious bank because it is a loan with a stipulated interest, which is prohibited usury (riba). It is permissible to take the amount deducted by the bank from the capital, which is five thousand, because Allah Almighty has allowed the one who repents from usury to retain his capital, and the mentioned amount is the entirety of his capital that the bank unjustly diminished. This is also permissible based on the principle of "al-Dhafar bi al-Haqq" (gaining one's right).