This is my response to you:
1. Capital gains tax need not be paid if the property is sold within the first 3 years of the purchase;
2. But in your case the capital gains tax will have to be paid;
3. Also the agreement should be registered within 4 months otherwise you will need to execute a deed of confirmation as well;
4. It is also the builder's mistake that he should have signed and registered the same;
5. You can sell the apartment only after the property is registered in your name;
6. You can also do this, tell the builder to sell the property directly to the buyer by executing a tri-party agreement;
7. This would save the builder from paying capital gains tax and the buyer gets a perfect title to the property.