Pagdi system-Monthly rent

Greetings, My Father has purchased a shop under pagdi system before 10 years in Pipe road,Kurla(west) near station and the said Pagdi owner of the land was taking some amount every month, which I guess is legal as a monthly rent inspite we have purchased the shop. As my father expired 2 years before, hence the pagdi landlord has called me last year and asked for the money. When i enquired and ask him to clarify he said for people residing or using the place he takes 600rs/month and who all gives that property on rent he takes 800rs/month, by giving receipt of 400rs only. When I questioned him that he is giving receipt of 400rs only and taking 800rs/month from us, he said this the rule and he is taking the same amount from everyone. I some how paid him as per his demand, now after 1 year again he is asking the same amount. My queries, 1) Is it compulsory to pay monthly rent for the pagdi property inspite of purchasing it. 2) Is there any rule stating that the Pagdi owner can take any amount and can give receipt of half the amount. 3) Also how can we approach BMC to take action or take the land under there custody? I will be very thankful, if you clear my doubts. Thanking you