Is it permissible to appraise an original sum of (2000 EGP) in gold in 1992, then calculate the value of this gold in 2012, and subsequently take this value from the usurious interest accrued on the sum which has reached 40,0 EGP, while disposing of the remaining interest?

If the bank is usurious, it is not permissible to deposit money in it with usurious interest. Whoever has done so must repent to Allah, and only their principal capital is lawful for them, for Allah Almighty says: "And if you repent, then you shall have your principal sums." As for the loss mentioned by the questioner, its cause is the depositor's choice and disobedience. What is required is to withdraw the deposit and take the principal capital, and to dispose of the excess interest by spending it on charitable causes for the poor and the needy. If the questioner is poor and in need, they may take from this money an amount sufficient for their needs. As Al-Ghazali mentioned in "Al-Majmu'", if someone possesses unlawful money and wishes to repent from it, if it belongs to an owner whom they do not know and they despair of knowing them, they should spend it on general Muslim interests or give it in charity to the poor. It is permissible for a poor person to take from it, and they may give it in charity to themselves and their family if they are poor.