Remedies against unlawful demand of investors

hi.My brother in law has recently purchased a flat which was purchased initially by the investor without registering in his name in his name then. We recently have done the registration on my brother in law's name. Now the builder's office people are threatening us to pay the service tax paid by the investor else they will cancel the agreement. Moreover the sales people of the builder's office are telling us to pay the tax on present agreement which is not acceptable. Please suggest what needs to be done.