Making a payment agreement

I have a question . My car was repossesed by Nissan in 2008. there were many irregularities in that repossesion because we make a final payment but they show as we didn't because that they reposses. They never said where the car is going to be auctioned. Anf after the auction I have a deficiency balance of 8285 dollars. At this point I want to make an agreement and make payments and be off of that as soon is possible and of course I never will buy a nissan car the problem is that they want that pay to an assigned collector I said ok but I need a validation from Nissan that they assigned the debt to that collector How I will make payments to somebody that is not my creditors. Nissan want that I make the payment agreement with the assigned collector but no way I no that it is no right I want to fix that but I want to be protected my debt continues being with Nissan no with the collector and I don't have a proper written documentation that proved me that this company has being hired by Nissan to make payments What I should do?