All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
In principle, an agency contract is lawful and an authorizer has the right to relieve the authorized at any time and with his own free will. However, if the agent has rights with respect to this power of attorney or it was issued to his benefit, then the authorizer can`t relieve him because doing so would harm him. "There are situations in which the power of attorney can`t be revoked, so it was stated in the book Al-Majma`: "The authorizer has the right to relieve the authorized so long as the rights of others aren`t tied to this power of attorney." {Al-Baher Al-Ra`iq Shareh Kanz Al-Daqaa`iq, vol.7/pp.187}.
Article (863) of the Jordanian Civil Code states: "The authorizer has the right to relieve the authorized whenever he wants unless other persons have rights with respect to that power of attorney or it was issued to the benefit of the authorized. In such case, it is unlawful for the authorizer to terminate the power of attorney or restrict it without the approval of the person/s in whose favor it was issued in the first place."
If the power of attorney is preceded by a valid contract of sale, then it is permissible to use it as means of authentication. However, if it was a mere power of attorney and no rights of other persons were tied to it, then it isn`t lawful to stipulate irrevocability because such condition violates the nature of the agent contract. And Allah the Almighty Knows Best.