Information regarding taxes while buying a property

I bought a flat in a 2-floor scheme for 60 lakh. In Feb 2012, I bought it with a verbal promise of possession in March 2013. The builder (against my wish) extended the flat scheme to 7 floors. After a settlement in July 2017, the builder then proceeded to give us a flat in the 7-floor scheme with possession in September 2018 and exchanged the file of the old flat for the new one. The new flat's AMC, AEC, Legal, Maintenance deposit, maintenance expense and taxes I'm liable for. The taxes have to be paid on 90 lakhs as that is the sum the jantri estimates in that particular area. Who is responsible for paying for the additional tax incurred? What documents does he have to provide in relation to this? When will the maintenance cost start, when the possession is handed over or when the BU Permission has passed? If not using the flat after possession, what should the maintenance expense be (other members will use the loading area but our built-up area won't be used)?