The ransom scheme has been around so long it is nauseating. It used to be people would steal dogs out of yards to collect the rewards. If the rewards weren't big enough, then the dogs were ransomed. This has now become a big internet business. Sadly, it works quite well. People will do almost anything to get their beloved pets back. Most of these jerks (strongest I can think of that the censors will let me use here) are overseas and use something like PayPal to collect the money. The chances of catching them is slim and, even if caught, the odds of a conviction are pretty much nil.
However, the idea that the fosters would steal the dog is just inconceivable. If they felt that way, they should have had the decency to either talk to Dude or call you, not just have the dog taken away. That's nauseating. The ransom scammers are vile people but if the fosters did this, they truly should be hung by their short hairs and have dogs nipping at anything that sags downward.