Flat Agreement

My query as follow:-

I decide to buy flat from reputed construction co. On the following Basis.

Booking/intial Amount                               Rs. 90000/-

After my 4th Floor is complete(slap)      Rs. 540000/-(Agreement time)

The flat choose by me is belong to their owner of Land. In this regard i want the following clarification.

1.    Is receipt from construction co. is vaild?.

2    What agreement i ask to construction co. or owner of flat?

Satish Paithankar