Permant alternative agreement

Hello Experts,

Small help and guidance required.

I am interested in buying one property in Mumbai.

1BHK Flat has been build in re-development project of old tenant society. 

Passion has been given to society 5 year back and society is also build.

Now,

I have heard that tenant of those building has to make Alternative agreement  (Usually builder has to do that) Without Alternative agreement person could not sale or mortgage for loan his flat.

1) Who should bare this Alternative agreement cost? (Myself-Buyer or seller)

2) What will be Alternative agreement cost for Mumbai--Chembur--323 Carpert Area.?

3)  Building secretary has told me that there is a option call "Alternative-Declaration”?

     Which will help in sales of flat and they have done in past by paying Rs 40,000/-  

     and will have no issue for me in getting loan.

Please help in clearing my doubt

Thanks

Abhi