Cancellation of sale deed

Myself is a land lord and given my property for devolepment vide Registered Agreement of devolepment and irrevocable GPA for construction of flats but the builder stopped the work in between i.e., after completion of structure and till then he was absconding and not in contact we tried hard to trace him but went in vein , now we came to know that he sold the flats which comes under his share 7 years back to some third parties on semi finished stage and these third parties move to court and got interim injunction against me for not to interfere in their peacefull posession and truly speaking they r not in posession since last 7 years and the building is in very bad condition and inhabitable any ways i have filed the appeal against the interim orders. intresting part is that i have never given any power of alienation to builder but still builder sold his share and was registered . now i want to cancel the registration executed by builder to third parties , therefore request you all members to suggest me the proper procedure to be followed as per law. thanks.