What is the procedure to get a collusive mortgage decree cancelled .

You can consult your advocate to know about the practical procedures for filing the proposed suit.

Facts posted are not clear to form proper opinion and oblige.

It would be better to consult and engage some other local prudent lawyer for proper analsying facts / documents of the case in hand, professional advise and necessary proceeding if you have lost faith in your lawyer.

If you are sure that you can prove that the mortgage decree was obtained by collusion, then you must read Section 44 of the Indian Evidence Act (old) or Section 38 of the new Evidence Act (Bharatiya Sakshya). 

This section of the evidence act says that we can show that a decree was obtained by fraud or collusion. This section would be applicable only if you can prove that the decree was obtained by fraud or colluion. 

You should not file a separate suit. You should file a petition under the petitions filed by your opponent (petition to set aside the sale and another petition) and explain why you think it is a collusive decree. That is all you need to do. If you are able to convince in your petition that the mortgage decree was obtained by fraud or collusion, then the court has to treat it as nullity and non est in the eyes of law. It will be forced to ignore the mortgage decree. 

The above is one option. 

I am not sure whether you can file another suit to set aside a decree obtained by collusion. A separate suit to set aside a decree is maintainable only if the decree was obtained by fraud. I do not know whether collusion is a valid ground to set aside a decree. 

The mortgage is a mortgage by deposit of tile deeds and not a registered mortgage created only to defraud my genuine debt .  The value of the mortgage schedule property in open market is only Rs. 6 lakhs by the date of alleged mortgage in the year 2013 but the plaintiff lent Rs.10 lakhs to the defendant in the year 2013 and filed a suit in the year 2019  for recovery of the mortgage debt and obtained a exparte preliminary decree against the defendant in the year 2020 and filed a claim petition in my E.P stating that he is a secured creditor to stall the sale proceeddings.  On the date of hearing of the claim petition in the year 2023  no Final Decree proceedings were initiated by the plaintiff and the claim petition was dismissed .  The plaintiff and defendant created a part payment receipt for Rs.1,00,000/- on 12/12/2022 and  filed Final Decree Petition  in the year 2024  which was again decreed exparte on 13/12/2024 .  Meanwhile I brought the schedule property for sale to recover my E.P amount and when the date of sale was fixed as 18/8/2025 , the plaintiff in mortgage suit filed E.P on 1/8/2025 and filed a petition on the date of sale  in my E.P under Sec.73 of C.P.C. for rateanbles or  to create first charge as he is a secured creditor. The sale of the property was conducted in my E.P and  I became highest bidders and the sale was knocked down in my favour and  I got set -iff  of my E.P amount wth he purchase money and I deposited the balance amount into the court.  Meanwhile again the plaintiff in the mortgage suit filed a petition for stay of all furhter proceedings in the E.P including confiration of the sale till the final disposal of his peittions .    

If your opponent filed his petitions on the basis of any decree, then you are entitled to show that the decree was obtained by fraud or collusion. That is the purpose of section 44 (old) or section 38 (new) of the evidence act / Bharatiya Sakshya. 

You can file your own petitions under his petitions (IA within IA) wherever he presented the decree, and present all the facts and any other evidence to prove that the decree was obtained by fraud or collusion. You can also file counter affidavits against his petitions to show that the decree was obtained by fraud or collusion. 

You should not file a separate suit to make use of the above provision. Wherever he presented his decree, there itself you file an IA within IA or counter affidavit or both to show that the decree was obtained by fraud or collusion.