Is the understanding that the Almighty’s saying: "And if you repent, then you shall have your principal sums" means that if a person repents from keeping his money in an usurious bank, then the entire amount withdrawn from the bank becomes permissible for him, even if it was mixed with interest that could be separated, correct? And what is the proper action concerning the remaining money?

If a Muslim is not in need of usurious money, it is not permissible for him to benefit from it. He should dispose of it by spending it on the welfare of Muslims. If he has used some of this money ignorantly of its prohibition, he must repent by regretting his action, and he is not required to donate a similar amount, in accordance with the Almighty's saying: "So whoever receives an admonition from his Lord and stops, then he may keep what he has already taken." However, if he was aware of its prohibition, he must donate a similar amount. It is preferable to donate all usurious gains, whether one was aware of the prohibition or ignorant of it, as a precaution and to absolve oneself. The verse "But if you repent, you may have your principal sums" means taking back the principal sums without any increase or decrease.