1) Your sale agreement will come under contracts act and your sale agreement terms will be final.
2) The validity of the agreement will be 6 months only.
3) The buyer should have intimated to you in writing and verbal notification is not valid.
4) Issue reply Lawyer notice.
5) You will get judgment in your favour only( you have done the registration of agreement of sale, you have waited for 6 months and you are ready to comply with your part (i,e refunding advance minus 50000).
5) Since your agreement has well defined terms ( amount specified, penalties vividly mentioned, time for completion of agreement specified), you should first file the suit under Order 37 CPC.
6) Don't wait for the other side to file the suit as he may want to drag on the case and create a scene that your property is a disputed property.
6) Suits filed under Order 37 CPC is called summary suit and is decided faster by the courts.
Order 37 CPC reads as follows:
Order 37 CPC of the best provisions in the hands of a proposed Plaintiff, wanting to institute a Civil Suit. Broadly it states as under:
Rule 1, Sub-Rule 2 makes it applicable to all suits upon bills of exchange, hundies and promissory notes or the ones in which a Plaintiff seeks only to recover a debt or liquidated demand in money payable on a written contract, an enactment, where the sum to be recovered is a fixed sum of money or in nature of any debt except penalty, a guarantee - in respect of a debt or liquidated demand.
Rule 2 requires an Order 37 Suit to contain among others, a specific averment that the Suit is filed under this Order and no relief which does not fall within the ambit of this Rule is claimed.
Under Order 37, there are two stages of getting the Suit decreed. One is at the stage of Rule 2(3) and the other is at the stage of Rule 2(6).
Rule 2(3) states the procedure for appearance of Defendant which is within 10 days from the service of the summons on him. After entering appearance, the Plaintiff serves on the Defendant summons for judgment within ten days from the date of service supported by an Affidavit; verifying the cause of action, amount claimed and that in his belief there is no defence to the suit.
Rule 2(6) states that in case the Defendant does not apply for a leave to defend, (a) the Plaintiff shall be entitled to judgment immediately or (b) the Court may direct the Defendant to give such security as it may deem fit. Sub-clause 7 states that in case sufficient cause is shown, the delay in entering an appearance or in applying for leave to defend the Suit may also be excused.
Hope this helps