1. You should issue a lawyer's notice through your lawyer to the builder to cancel the agreement for sale of the flat since it is only 'Agreement to Sell' and not 'Sale Deed/or Sale Agreement'.
2. The refund of money depends on the terms of the agreement. Since the agreement provides for a forfeiture of 10% of the flat value the builder may apply this clause against you.
3. It is not permissible for the builder to delay remitting 10% of the flat value to you till he finds a prospective buyer. The builder cannot travel outside the four walls of the agreement. If the agreement permits the builder to refuse to refund the balance amount until and unless he finds a new buyer then such clause is illegal.
4. Issue a formal lawyer's notice to the builder to cancel the agreement and seek the refund of 10% of flat value. If builder does not refund your amount then you may file a lawsuit for recovery against builder in the court.
5. It is recommended that you engage a competent professional lawyer who can assist you with all this.