He has repented from dealing with a riba-based bank but he does not know what his capital was
Question

                I have a friend who used to deal with riba-based banks, and he took a loan and engaged in other riba-based transactions – we ask Allaah to keep us safe and sound. Allaah blessed him with guidance and he stopped all his dealings with the banks. Now he does not know his original capital, and he cannot separate it from the loans and the interest. He wants to purify himself and his wealth from previous years. What should he do? Please advise us, may Allaah reward you.
Praise be to Allah.Your friend 
    should contact the bank to find out how much the interest was that was added 
    to his capital, then he should get rid of this interest by spending it on 
    charitable causes. See question no.
    292 and
    2370. 
If he does 
    not know how much it is exactly, he should pay what he thinks is most likely 
    to be the amount of interest that he took, and Allaah does not burden any 
    soul beyond its scope. 
With regard 
    to the interest-based loans that he took and has paid off, that belongs to 
    him. It is sufficient for him to repent and regret what he has done, and to 
    resolve not to repeat such haraam actions. 
We praise 
    Allaah for helping him and guiding him away from these haraam interactions, 
    and we ask Him to bless him in his wealth and to provide for him of His bounty. 
And Allaah knows best.