Zakaah is obligatory upon the lender, not on the borrower
Question

                I took 5000 from a man, and when I had taken it and spend it on my needs, he said to me: You have to pay zakaah on the money that you took from me. I said to him: It is not permissible for me to pay zakaah on it. He said to me: Give it back so that I can pay zakaah on it myself. But I could not afford to do so at that time, so I had no choice but to pay zakaah on it. Is there anything that I have to do in this situation?.
Praise be to Allah.
    What this man has done is 
    haraam for two reasons: 

    1.

    He has asked for something 
    additional to the loan. It is not permissible to take something additional 
    to a loan or to benefit from the loan, because it is a contract that is 
    based on kindness, seeking thereby reward. So it is not permissible for the 
    lender to take from the borrower something that is additional to the loan, 
    or to impose any obligation on him or to stipulate anything for him, because 
    this is riba, and in fact it is the worst kind of riba. The Prophet 
    (blessings and peace of Allaah be upon him) said: “Every loan that brings a 
    benefit is riba.” Even though there is some debate concerning the soundness 
    of this hadeeth, the scholars are unanimously agreed on this point, that the 
    lender is not permitted to benefit from the wealth of the borrower because 
    of the loan. 

    So what this man has done is 
    riba, and he has to repent to Allaah and return this additional amount that 
    he took from you and imposed on you. 

    2.

    The zakaah on wealth is only 
    due from the owner, not from anyone else. The fact that he forced you to pay 
    zakaah does not mean that his duty has been discharged, because zakaah is 
    due from the one to whom the debt is owed, and he is required to pay it. End 
    quote. 

    Al-Muntaqa min Fataawa 
    al-Shaykh Saalih al-Fawzaan 
    (2/304)