Hello Rahul,
If Rs.15,00,000 is the cost of acquisition at the time of sale (indexed value), than you are having a long term capital gain of Rs. 5,00,000 today. Now you had invested whole Rs.20,00,000 in another property and than sale the same at same cost, so now your previous exemption is withdrawn and now you had a short term capital gain of Rs.5,00,000. Well you had to pay tax either this way or that way.
In my opinion, it is better to pay short term capital gain rather than long term capital gain means invest in other property and than sale. As of now you had to pay STCG @ 10% where you also claim deduction u/s 80. And if you opt for LTCG, than you had to pay taxes @ 20% flat.
So this is also a nice tax planning for tax savings.
Thank you