Releasing Fund against Demand letter got from Builder

Hi Sir / Madam, i have booked flat in pune that is on 3rd floor. Actually building is 20 floor planned. So far builder has only 19 floor approval with him, We have already given him 80% money. as my flat is on 3rd floor he completed brick work and plaster and he requested me to provide him again 10% money (means almost 90% fund) and he has mentioned date of possession is Apr -2018 that is almost 1.5 years far from today. I requested him to show me 20th floor Approval and then we can release fund, but what he said that whatever mentioned in your registry , as per that after brick and plaster work you have to give us money irrespective when we are giving possession to you even if it is on ""Apr-2018. Sir we have already given him 80% and just want to see 20th floor approval. We stop his payment now, he is threatening us that if we will not pay then we have to give additional interest on actual payment. Sir/Madam Request you to suggest us something as per defined law. -Manish thakare