ECU(computer box) swapped

I'm disgusted by the service I received at VW McCarthy Durban, my 2015 Golf GTI had a misfiring and had loss of power. I drove it to their dealership for repairs and was told a ridiculous amount when it was stripped, I didn't agree on the price so I opted to take it as is because I don't have that kind of money. Asked the service advisor if no parts were removed or swapped from my vehicle as it was stripped and been with them for a while, she said NO. Towed my car to another VW dealer, upon assembling it, he asked me to buy parts that were missing, parts that should've been there because I drove the car there, I have all the receipts as well. When the car was finished my mechanic started the car, it idled for 2 to 3 seconds than cuts off, he checked and found that my ECU (computer box) is not my original as the chassis number didn't match my car, he said it has been swapped. We called and enquired by giving them the chassis number on ECU on my car, they told us that the vehicle was at their premises the same time as mine. They also said that car was at their premises for a battery change and has left for JHB. Now why would you take my ECU(computer box) and put it on another car? When we called again asking how am I going to get it back? The story changes and they are saying they didn't change anything yet they have all the records with them. Manager is always on meetings when you ask for him as well. I need my ECU back, the chassis number tell it all, this has caused my heartache and I wouldn't advise anyone to buy a car there or take it for service as well cause they change your original parts. VW McCarthy Durban. Mind you I bought my car at the very same dealership and this is how they treat me and this is the thanks I get. Cell : ***