My question involves an auto loan or repossession in the State of: Louisiana

I went to a dealership in Metairie, LA to purchase a car. The salesman said he'd only make $250 on the sale, so he asked if I'd cut him a side check of $500 he'd knock $1000 off the price of the car.

It sounded like a good deal so I said ok. I wrote the salesman a check out for $500. I then wrote a check out for the downpayment for $5,500. The salesman edited the check I gave him for $7,500. (He added a "7" in front of the 500 and the word seventy on the second line of the check). He then gave the check to someone to deposit in their account. That person had the $7,500 in their account to cover the check so it was cashed.
My bank has since then returned my money to me but I don't know what happened to the salesman.

Now when the downpayment check went to my bank, it bounced.
From there the dealership harrassed me by sending the same salesman and his manager to my house to take the car back on more than one occassion. The local police told them they have to send the check to the District Attorney and file the necessary paperwork, and they did not.

The final draw was when they sent some man to my door in a red jumpsuit showing a police ID about 10pm that night. He threatened to tear down my garage door if I didn't voluntarily give up the car.

He had no paperwork for me nor did I sign anything.

I gave him the car.

Was it legal for the dealership to operate in this manner? What and where are the laws to support this?