Suppose someone asks his friend for a loan and he says "sure, no problem, but there is interest that comes along with that". Later, the borrower repays just the principal and says "the Torah forbids interest so I am paying only the principal back; even though I agreed initially to interest, I just said it so you would give me the loan".
Is there an halachic obligation to pay the interest? (The lender can claim he would have wrote up a heter iska agreement instead.)