Sale of neighbors property involving common wall and entrace

My neighbour has sold their property but has not given us any intimation prior to the sale, and is piling us with this dilemma by making us deal with the new buyer of the property. Initially when i had my old owners there was a one brick partition between ours and neighbors wall.While the old neighbor Father has a discussion with us where he wanted to close the one brick partition as there were seepage's in his house during the rains. My father agreed to his saying and allowed him to close the space ,being good neighbors. In the course of action we also had a warander for our house which was enclosed previously and the neighbors approached my father to have the enclosure to be extended from their end to ours having one common space as they had little children so we agreed to keep the space for them also to use. Now my neighbor has sold this place on a quiet and is vacating bringing to our notice that the new entrant will be staying in a few 2-3 days time. We have to make our walls separate and enclose my warander again but he is being oblivious to what we have mentioned to him. I have all my property papers in place i would like to know the best solution in such a scenario. Thanks & Regards Sherwin