Resale of apartment on the basis of allotment letter

I am looking to buy an apartment in Mumbai. The seller X has the allotment letter provided by the builder but has not entered into an agreement with the builder. I am told that builder will agree in-principle to the transaction at the price arrived between the buyer (me) and the seller X. Please answer my following questions: 1) Legally, who would be the seller - the builder or the person X on whose name the apartment is allotted? 2) To whom I should pay the token amount? 3) Would I get a home loan on the basis of allotment letter? Who would be the beneficiary? The builder or the person X? As I understand, since the agreement has not been entered into the title of the property is in the name of the builder. And so the answers to the questions above will be the builder. If so, what is the role of person X in this transaction? This is all the more important since my negotiation regarding the apartment are primary with person X. The builder will enter the picture only once the price is finalized. What are the things I should keep in mind while talking to person X about the apartment?