Returning Money Taken Without Permission
Question

                A person has given me money to put in my saving account in an Islamic bank, and then he will take the profits. I needed some of this money and my intention was to estimate the amount of profit of the amount of money I used, and add it to the profits that will be given by the bank when the time of distributing profits in 3 months comes, and give it all to him. Is this permissible? Does it include riba (usury) at all? This person does not know anything about this, and I am giving this to him happily considering it a gift?
Praise be to Allah.
    Firstly: 

    You did wrong when you took the 
    money without your friend’s knowledge, because it was a trust (amanah) that 
    was given to you, so it was not permissible for you to transgress against 
    it. By transgressing against the trust you became liable for it, so you have 
    to return the same amount, but you do not have to give anything more than 
    that, unless you took the money and profited from it, in which case the 
    profit must be given to the owner of the money. This is according to the 
    view of some fuqaha (jurists) who think that the profit on usurped wealth 
    and that which comes under the same ruling should not be taken by the 
    usurper, rather it belongs to the owner of the money. You may tell the owner 
    of the money what has happened, then agree on the way in which the profits 
    are to be shared out. 

    See also the answer to question 
    no. 
    87695. 

    If you took the money and did 
    not make any profit from it, then you must return the same amount, and there 
    is nothing wrong with giving something extra as a gift. 

    Secondly: 

    You have to repent to Allah for 
    transgressing against this trust. The conditions of repentance include 
    resolving not to do it again, because repentance is not valid if you 
    continue taking your friend’s wealth without his knowledge.  You must also 
    tell your friend and ask for his forgiveness, unless you are afraid that 
    telling him will lead to negative consequences, such as his cutting you off, 
    in which case it is sufficient to return the money only. 

    And Allah knows best.