So ...
They employed you as a pilot and 6 months before you resigned, solely at their behest, they opted to train you, at your expense, on a different aircraft type that they would have already known that they were selling.
So what did you do for the months after your training and prior to your resigning if they didn't have an aircraft for you to fly ... Sit on your butt?
... Is a pilot not employed to fly an aeroplane?
In what jurisdiction are you, USA or France?
As they say possession is nine tenths of the law, you have your money and if they want to take it from you then they need to take you to court and given your story, as you explain it, I think they'd have a very hard job to persuade a court in their favour.
P.S. And warn all others not to touch this outfit with a bargepole.