In addition to the answers provided by other experts, I add the following for your information:
1) The sale value is Rs 90 lakhs and the sale proceeds you will be receiving is net of 1% transfer fee on sale proceeds. So your net sale consideration will be Rs 89,10,000/- only. Your actual cost is Rs 80 Lakhs. So your capital gains will be Rs. 9,90,000/- only. Now whether it is Long Term or Short term depends upon when you actually acquired the asset. The letter of allotment gives you right to get the property upon the payment of sums as per schedule given in the letter of allotment. You may stretch it to say it is Long Term Capital Gains, if the date of letter of allotment received is more than 3 years from the date of transfer to the new buyer. Even then, your payments may not be in full at the time of time of receiving the letter of allotment and the actual payments might have been spread over the period from 2013 to 2016, till you were given offer of possession. So there is not much benefit in indexation. Whether the amount of Rs 9.9 Lakhs is a short term capital gain or long term capital gain depends upon how the documentation is done.
2) As the new buyer will be getting transfer of rights from you on the flat booked by you and you may not be party to the sale deed between the buyer and the builder, he needs to deduct tax on the payment made to the builder u/s 194L, as the builder is a resident. However, as he makes the payment to you as a Non Resident, he also need to deduct tax u/s 195, applicable to Non Residents. Otherwise, you will not be able to transfer the sale proceeds to your NRE a/c or any other account in USA.