Procedure to make final payment as written in sale deed

Dear sir, I purchased a flat in Mumbai and paid 80% of the amount to the seller and requested him to take balance 20% after sale deed gets registered that is registration fees and stamp duty paid at the sub register office. The seller agreed to it but requested to pay 20% after 30 days and before 45 days after the documents/sale deed signed at the sub-registrar office. He has included this point in the sale deed that he will give the keys and position of the flat when the final payment of 20% will be done. My 3 questions are: 1). Does the above point /clause affect the sale deed. 2).in the future when I want to sell the flat will that clause in the sale deed make a problem. 3). Once I pay the final 20% what documents should I take from him to avoid problems in the future, as we have already signed the sale deed 3weeks back. Note: I agreed to it because he said he wants time to search a flat on rent and shift so that he can give me my flat. He wanted time to search and shift and the pending 20% I can keep till that time. That was his interest, but I agreed to it for humanity sake, then I left to my native town as I am not from Mumbai.