What can I do if someone sells property with my name without my knowledge?

Sir, the house building is constructed by a builder through collaboration agreement. Building owner was expired. His wife along his 3 sons are the legal hires. As per the agreement the builder share belongs 1st and 2nd floors. Now, the builder prepared a Power of Attorney document in which he wrote some additional thongs which are not included in our collaboration. Objectionable point is that he quoted in our name and on our behalf. Can sell his flats to anyone in our names.? We need you help in this matter. Can you please some outlines of POA.