Tax dues and possession letter for home loan.

Hello this ia Sougata Sarkar from west bengal. I am goinig to purchase a flat from land owners side. He has provided me all the documents required for home loan which I am going to take from my employer. He has provided land deed,link deed ,development agreement ,power of attorney legal heir declaration, tax receipt of KMC for quarter 1/2019-2020, 2/2019-2020. But as I see tax is not upto date and approx 25000 tax is due which I have told him,but he said that this part is of developer. Meanwhile my employer has demanded for possession letter on developer's letter head before disbursement of loan.This also I have conveyed to land owner. He agreed to provide. Please advise shall I input this tax matter to the agreement? Do I have to borne the due tax in pro rata system, if I do the registry before the tax paid? Or I have to proceed for registry only after the CC is handed ? The property is 80% completed, near about 3-4 months left for overall completion.When can the Possession letter be handed by developer to land owner- before or after getting CC? Please advise How will I proceed.?