Registration and Stamp Duty on Pre-Construction Property

Hi. I am an NRI and have purchased a pre-construction property in Mumbai. I have paid 26% of the agreed value of the home and have an allotment letter for the unit. I want to get an agreement made between myself and the developer (primarily because I want the developer to sign off on more details/terms of purchase like fittings and furnishing). I am told/read that to have an agreement, the property needs to be registered and I need to pay stamp duty. Can you please advise if this is the law? Or can I have an agreement with the developer without registering the property yet (since it is not complete). The reason I ask this is because the stamp duty is a sunk cost. Tomorrow if the builder does not complete the project, even though the agreement might help me recoup money, the large stamp duty will never be recovered from the government. Also, is it also necessary for to register the property since I have paid more than 20% of the agreed costs. Is there some Maharastra law stating that? Can there be another signed document between myself and the builder with regards to fittings and furnishings that are not a part of the allotment letter? What is the value of such a document? Appreciate your help. Thanks.