Does anything remain binding upon the questioner after she took a sum of money from a man, and the man informed her of the obligation of zakat upon it, then she asked to return it to him so he could pay its zakat but she did not find it, so she paid its zakat herself?

What this man did is forbidden for two reasons: First, he demanded an increase on the loan, and this is usury (riba), because the Prophet, peace and blessings be upon him, said: "Every loan that draws a benefit is usury." Therefore, he must repent to Allah and return this increase. Second, the Zakat (definition) on money is obligatory upon its owner, and his forcing you to pay the Zakat does not absolve him of his responsibility for it.