What must the questioner do to atone for his sin after purchasing by installments from a company that later turned out to deal with a usurious bank, and is it permissible to give the amount of the interest as charity?

So long as the questioner was unaware of the true nature of the matter, we hope that there is no sin of riba (usury) upon him; based on the Almighty's saying: "And there is no blame upon you for that in which you have erred, but [there is blame for] what your hearts intended." If he fell short in asking and inquiring, then let him seek Allah's forgiveness. If he can request annulment and cancellation of the contract, then let him do so. If he cannot, then Allah does not burden a soul beyond its capacity, and let him seek Allah's forgiveness. What he purchased with this loan is his property, and he may use it for permissible things, because the ruling on riba relates to his liability, not to the specific asset itself. As for giving charity to atone for that, there is no harm in it, from the perspective of performing good deeds, not as a substitute for the usurious interest.