1) builder cannot cancel agreement arbitrarily and sell flat to third party
2) the mistake you made was you should not have agreed to change clauses form slab wise to date wise payment
3) it is necessary to peruse agreement entered into with builder to advise
4) further it is necessary to peruse case filed by you against builder in court to advise
5) most probably you would get refund of money paid by you with interest and compensation for mental torture undergone by you