Agreement of renunciation

Hi, I am in the process of buying a property in Bangalore which is still not registered, The seller ABC has sale and construction agreement in his name and there is also a document called "agreement of renunciation" which reads that Renouncor (ABC) was allotted the flat by builder and he (ABC) approached to Renouncee (XYZ) for assisting him in payments for the full initial and all subsequent amount towards builder and offered to renounce his rights to register the flat to renouncee (XYZ). The last line reads as the renouncor (ABC) does not have any rights on the scheduled premises and the flat shall be registered in the name of renouncee (XYZ) My question - Is it legally ok to buy such property wherein the origianal allottee ABC has done sale and construction agreement with builder and subsequently done the agreement of renunciation with somebody , in this case XYZ, So legally who is having the ownership rights? is it Mr. XYZ. Another query -- is Mr. XYZ authorized to assign his rights back to builder so that the property could be registered in my name as so far it is not registered.