In your case I think still financier is not aware of private sale agreement therefore he is threatening you of filling case u/s 138 of Negotiable Instrument Act. As & when the financier gets the information/copy of your private sale agreement, he will straight way file criminal complaint against your company. PL NOTE THAT EVERY FINANCE COMPANY WHETHER PRIVATE OR GOVERNMENT ALWAYS DEALS IN MONEY, MONEY AND MONEY ONLY. THERE IS NO BARTER SYSTEM EXITS IN CURRENT FINANCE SCENARIO. You may approach financier and obtain due written permission for sale of vehicle. You shall have to deposit difference amount after crediting the sale proceeds.
First of all you should take the vehicle in your possession and then approach your financier so as to gain the trust of your financier.
If vehicle had a loan,then NOC of finance company should have been procured.
Whenever a vehicle is sold intimation to RTO for change of ownership should be filled up and ensured that name change is done.
You have landed up into a problem because basic legal provisions have not been kept in mind while selling the vehicle.
Speak to the purchaser and ask him to take back the money and take the car back.
You have literally no legal recourse against this person.Stamp paper agreement is of no meaning in law.