Builder demanding cash for dry areas for a agreement signed in 2006 and after delaying the project for 6 yrs

Q: I signed an agreement in 2006 with one builder for a project in Goregaon West .
Project got delayed for 6 yrs . The orignal builder tight with a new bigger builder to complete the project .Now the project is complete and they want us to sign a supplement agreement under new builder name which includes flower bed and dry areas . The amount to be paid for these extra areas is shown zero on the agreement ,but the old builder who still partner with the new builder and who signs the agreement on behalf of big builder is demanding cash from his buyers for the extra dry area and flower bed .
Without Signing this agreement we wont get possession .
This builder is also holding my cheque payments which i gave for the completion of the project .
Our agreement was already signed in 2006., hence they cannot charge any more money and that too after 7 years delay of the project .
kindly advise what should I do