If you give a power of attorney to someone and the person misuses it and also lies about how it was used, what does the sharia say about that?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger. 

If this person to whom you gave the power of attorney acted in a manner in which he is not authorized to act, then he has betrayed the trust and is sinful.

As regards what he is obliged (or required) to do in this case, or the consequences of his actions, then you should clarify the matter to us and provide details about what exactly happened.

Allaah knows best.