Parking allotment

Dear Sir, we had booked an apartment in 13th floor in a RERA registered property . Initially the property was 11 floor sanctioned but later builder got permission for of construction for further more 3 floors total of 14 floors. There are set of 14 floors in each building namely A, B, C, D. ( phase 1 ). Builder has obtained Completion certificate. As per builder he extended the storey of building from 11 to 14 floors due to which he has shortage of covered car parking when we insisted he agreed for a covered car parking but we had to pay him amount which he cannot show in agreement anywhere or nor gave a receipt of it. The builder has mentioned in agreement a covered car parking with its measurement but has not allotted the number for parking. He says he will put a board on covered parking area but cannot write slot number in agreement. He has mentioned that the parking is selected selected by us in first come and first serve basis ; also in future if there is a dispute for parking among society then builder's decisions of parking allotment will be final. Also we had given them an acceptance letter of covered car parking as per builder's request which they call as allotment letter having builder's Stamp saying that we have booked our flat number---- for which we have selected one covered car parking measurement ---- and parking number ----- on first come and first serve basis. In future due to any technical reasons if I have to select some another car parking , I shall select from the options available at that time for which I am aware and agree to it. Now I have a query whether this acceptance letter which they have stamped with above mentioned details is really an allotment letter or not.If not then what should be the format of the same. Will these clause in acceptance letter lead me to future problems regarding covered car parking. Secondly in future will there be any problems with parking after society formation. I fear if I have to loose car parking and money due to problems in future. Can you give solution as to what should we ask builder so as to safeguard against societal car parking issues. Kindly please suggest a solution to this.