Get a lawyer, eminent domain is not something you can deal with yourself. I am not a lawyer, but I have had property taken by the government. It can be a very frustrating and unfair process, but the government does have the right to do it. What I learned was that it is very hard to fight the taking of your property, but you do have a good shot at getting more money if the offering price is unfair. There are sometimes other costs the government will pay like relocation and for loss of business that depends on your local laws.
If you really want to keep your property one strategy is not to negotiate or deal with the government, force them to go through condemnation to get your property. Go to the newspaper and local news folks, try and get some sympathetic reporting. It can make an interesting story. There is some chance the government may not go through with it if you make it hard and embarrassing for them. In the end if this strategy fails it will not reduce the amount of money you can get.
Good luck.