Is it alright to pay 40% of the value of property without any agreement with Developer

Q: I am interested in buying a 3 BHK flat at Emami city, Jessore Road,Kolkata. On enquiring at the office of NK Realtors(who are carrying out the construction on behalf of Emami group), I came to know that they are asking to deposit Rs 2 lacs for booking and at the same time 30-40% of the base value of the property (which amounts to about Rs 30-40 lacs) according to the progress/stage of construction of different buildings of the project. After payment of this amount, they are giving only an allotment letter but not concluding/making any agreement for the property. My query is that an allotment letter is hardly any legal document whereas an agreement is legally binding and when I am committing such a large amount of money (30-40 lacs), I find it highly irregular that such a big group like Emami is not making any agreement while taking money. Is it alright to go ahead with such an investment ?