Purchased truck paid amount but Owner is not giving NOC paper

We (buyer) contacted a company who want to sell truck, we inspected trucks and then we visited seller company office for meeting to buy trucks on discussion we were close to deal but deal was not done due to 2 lac diffrence in deal. later two days after on telephone deal was done with seller on 59 lac for 4 trucks. in this call we decide to the terms that we have to pay loan outstanding amount 51 lac and we will get delivery of trucks and seller will pay the amount to bank and release NOC Paper and we(buyer) have to pay the remaing amount on receving paper. We have recording of this entire conversation. In line to above discussion seller sent us performa invoice and asked us to pay the loan outstanding amount. which we pay them by bank RTGS. and then after took delivery of vehicle. But seller didnt deposited the lon amount and not give us NOC paper. since last 4 month seller has not paid loan but regulary paying bank installment but not giving us NOC paper. Now seller is asking their trucks back but saying he will return the money. As we transported truck from long distance and it was old trucks we have spent approx 20 lac for transportation and maintenance. we dont want to give the truck back to them. we want NOC paper from them. Please suggest legal way to get it.