Parking Issue in society on extra space alloted by Builder

Hello sir, I bought a flat in Pune , registry done in 2018 and possession in 2019, after possession the parking allotment was done and the space that was allocated to our flat had an adjacent space which was covered from two sides (power room wall and stair case wall), since it was blocked space and he could not allot it to anyone else. he asked us to buy that area so that we could use it as extra space in our parking, We agreed and paid him 1 lakh by NEFT in builder's account. But at the time of receipt he did not mention it as parking amount, although he gave us one print out which was the parking layout of our building and there it was mentioned our flat number on both the space (originally allotted and the one that we bought). Now after society formation the society members are asking for the proof of this space belong to us. Please suggest and help how to resolve this issue. Please note - the extra space is not public space ( its a corner covered from walls from two sides) and cannot be used by any other residents.