In case such a Waqf is carried out during your lifetime, it is impermissible for you to sell the concerned property or change the assignment thereof.
However, the ruling on Waqf which is conditional to the death of the endower is similar to that of Wassiyyah (will) i.e. it is permissible to withdraw it or move it to another property during the lifetime of the bequeather. Finally, the condition that the endower should act as the attorney during his lifetime is valid. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.