Loan Closure for vehicle

Hi, My self Dev Pandey from Mumbai. I had purchased a heavy vehicle 3 years back and took loan of 7 lacs from Cholamandalam. But due to bad market conditions was not able to get it running for long time and not able to pay 5 EMI from the bank. Due to which they took away the truck. At that time i have an outstanding principal of around 5.75 Lac. After taking away the vehicle bank never contacted me. Few months back i started getting calls from some staff from the bank asking for some dues pending with the bank. When i asked about the pending amount due, they said the bank had sold the vehicle for 2.5 lacs only. I was bit surprise with this sale amount.This amount is far lower than its market price(7.5 lacs) and also the pending due loan on that vehicle(5.75). So now they asking me to pay the pending due of 5.75-2.5 = 3.25 lacs as communicated by the local branch staff. When asked why they sold at this price, they do not reply anything, neither they show me any paper that it was sold at that price. And also as far as i know they cannot sell that vehicle below the reserve price of 5.75 lacs. When i tried to clarify this matter with the cholamandalam customer care, he says the loan account is already closed with no due left. And they also send the NOC document on 17th may 2014 to me which i never received. For duplicate NOC copy and loan closure document they say to collected from Local branch office, and branch office is asking to clear this due of 3.25 which looks ill logical to me. Sometimes i think the branch office is just trying to fool me and make huge money since they also ready to negotiate for this price to 50% off. Can some one guide me what action can be taken to resolve this issue. Can the bank sell the vehicle at price lower than 50% of the reserve price or 75% lower than market price???