Agreement for sale

sir, we have two agreement for sale for 2 flats in an incomplete building, one agreement is registered whereis one agreement is non registered simply made on a single 100/- stamp paper, both the flats full payment has been made and receipt issued by the builder and possession was handed over in 1998 itself, the builder did not complete the building and passed away without and legal heirs to the property, the land originally belonged to one person who made a non registered sale deed in 1987 in favour of another builder and received payment for the the property and issued the builder a power of attorney, the builder was not able to complete the project so they roped in this new builder who made complete payment to the old builder and have taken a deed of assignement of the property in their favour, this assignment deed is also signed by the original owner in 1994 the original owner passed away in 2010, and now their legal heirs are trying to claim the ownership on the property and trying to sell off the property to some goon builder who is saying he will vacate all the owners ( by way of agreement and possession receipts ) who are in possession can someone please advice what can be done, we have filed cases against the legal heirs who are trying to dispose off the property, but have still not got a stay stopping them to create theird party interest in the property