Property related dispute

Dear Sir/Mam, We had a land in  and we wanted to construct full house (3 storeys) and we got an offer from builder that he will construct 3 floors (Ground+1st and 2nd) for us free of cost in case we surrender one floor to him. So we agreed on the terms and surrendered 2nd floor of the building on the terms that we will also provide separate car parking to the second floor owner. So it was decided we will have Ground+1st floor+roof rights of the building and it was also mentioned in the registry that 1 meter of passage will be used by as a common area for entry into the building. Now, that builder sold that 2nd floor to other party and the owner of 2nd party is showing his allotment letter stating he has right to 1/3rd of the area of the property and needs 1/3rd area for parking his second car on our ground floor apart from that 1 meter passage which was mentioned for common use. Kindly advise, what should be done in this case.