This is one of those areas where lawyers get involved.
Are you in a nation/state that operates under rule of law? Read the fine print in your contract, there has to be specific language that gives them the right to keep your registered domain. Do the laws under which businesses operate there allow them this right?
Are you a registered business, how long have you been in business and is your business name a service mark or trade mark? Law may give you leverage against theft of your business name.
You have 50 businesses, therefore a sizable warchest and body of people to pull from. You don't have to do it alone. Don't skimp by hiring a rubbish attorney's office.
Stack Exchange cannot give you the specifics, an attorney well versed in your local/state/national law is the only entity capable of giving you that advice.
And next time around, have someone knowledgeable in looking for gotchas look through the contract before you sign it.
A sign that was on the wall in our automotive shop, pertaining to cheap parts, has application everywhere: "The bitter taste of low quality is long remembered, the sweetness of low price is soon forgotten."