If the sale agreement was not made on a registered document then you cannot enforce it in court of law,, however you can get refund of advance amount with interest but for that you have to state strong reasons before court.

Besides for this Rs. 1.5 Lakhs, you may be spending a huge amount towards litigation expenses and lawyer fee,  which may be more than the amount he is ready to return to you  at this time and also you may have to spare your valuable time, energy, also all through the pendency of the case you will be mentally disturbed as well. So you can think about it properly and take a wise decision at right time. 

The whatsapp messages are not considered as evidence by court of law. 

Terms of agreement for sale is binding on parties 

2) it appears seller does not have clear and marketable title to property 

3) mutation of property has not been done in seller name and his khata application has been rejected 

4) you are entitled to full refund as to the is not clear and marketable 

5) issue legal notice to seller to refund your money with interest 

6) if he refuses sue the seller to recover your money with interest 

1. If it's a registered Sale Agreement it will be valid for 3 years from the date it's executed.

2.  Electronic evidence will be admissible, such as whatsapp, etc, subject to certain conditions.

3.  It depends on the clauses mentioned in the Sale Agreement, but nevertheless it can be challenged in the competent Court.

1. There has been default on his part on many counts right from wrong Survey No. to Kfhata problem. He cannot take advantage of his defaults.
2. Whether agreed sale consideration is 43L or 45L will not affect your case as you are seeking refund of advance paid.
3. it also amount to fraud and cheating by him as there were defects fin tiled. You can also file a criminal complaint against him for cheating.
4. Issue him a lawyer's notice with particulars of all defects of which you were kept in dark, seeking refund with interest  and cost of notice seeking refund within 15 days, after that file suit for recovery in Court.
5. His confidence is misplaced. It is false bravado.

Dear Sir,

My answers are as follows:

1. Is the sale agreement is still valid can be the proof of the sales process?

Ans: Any sale agreement is valid for three years.

2. I don't any proof other than WhatsApp chats that we have negotiated to 43 L for the property so is WhatsApp chat is admissible in the court?

Ans: It is okay, you can cancel the agreement at any time since there is lot of delay due to fault of the vendor. You can mention the same and get the legal notice for refund of your amount with interest.

3. Is there any way I can get back my full advance?

Ans: Yes you can get back entire amount with interest.

Dear client,  I am sorry to hear that but in this case you have no remedy but to follow the contract which basically talk about 45 lakh. Since your sale deed agreement mentions this amount, you are obligated to pay it in case of any legal dispute. 

However,  if you want you can cancel the sale deed citing issues that you have faced. 

It's true that whatsapp chats are considered as prove under the eye of law but the situation in hand is tricky and it might create issue for you.

Issue seller legal notice to refund your money and to cancel the registered sale agreement 

2) if he fails to respond file complaint against seller before consumer forum and seek orders to direct seller to refund your money with interest ,to cancel the registered same agreement,claim litigation costs and compensation for mental torture undergone by you 

3) seek injunction restraining sale of property by seller 

The registered sale agreement can be cancelled only by executing a registered cancellation deed executed jointly by both the vendor and vendee. 

2. Yes you were advised to issue a legal notice in this regard communicating your decision to cancel the sale agreement for the reasons you rely upon. 

3. You can file a suit for specific relief of cancellation of sale agreement and return of sale consideration amount and in the same suit you can file an application seeking injunction to restrain him from selling or alienating the property in any manner till the disposal of main suit. 

1.  To cancel the sale agreement, send a legal notice to the prospective seller to get the property registered for Rs. 43 Lakhs, as agreed , within the time agreed upon in the sale agreement otherwise let him agree for cancellation of the sale agreement in the jurisdictional Sub Registrar's Office, where it was registered, returning your advance amount of Rs. 3 Lakhs. In case the prospective seller refuses to cancel and return the advance amount, file a case in the jurisdictional competent Court for specific performance.

2.  Before initiating cancellation of the sale agreement, send legal notice as stated in (1) above.

3.  It's the responsibility of the seller to furnish all the necessary documents, such as Khatha Certificate & Extract, Latest Tax Paid Certificate, etc..at the time of registration of the property in favour of the prospective buyer.

4.   You can bring Injunction Order from the competent Court restraining the seller from selling the property, till he returns your advance amount.

Dear client,  It is possible to cancel the registration of your conveyance or title deeds (sale, gift, exchange, transfer) by registering a cancellation deed. A cancellation deed is a legal document that extinguishes any rights the buyer of a property may have had to it.

cancellation is possible when and if:

An individual feels that the deed is voidable or has a doubt that such a deed will cause him injury if left outstanding.
If the deed was registered according to the laws prescribed in the Indian Registration Act, 1908, the court shall send a copy of its decree to the officer in whose office the instrument has been so registered; and such officer shall note the facts of its cancellation.
The cancellation may also be executed by mutual consent of all parties.

Moreover,  he can seek relief against the cancellation.

And yes you can seek stay order from the court.

By filing cancellation deed. 

If the rights are related to said agreement then after cancellation you can't

Yes you can ger stay