I have an opportunity to purchase an old and rusty bonsai pot made of Cast Iron. But is cast iron dangerous to the tree's health? If it is can it be treated so it will not be dangerous? Is there any other information I should know before making my decision?

Welcome, Oyster Sauce.
I have never used one, but there is some controversial info about this subject.
From this website:
"They may be made out of ceramic, concrete, plastics and certain metals (metals may release toxins) and one can make their own pots. But what is considered a classic Bonsai pot is that it is made of ceramic or porcelain, and that it is stoneware burned, which means that it absorbs and holds no water in the material. It is important for the health of trees.

"
Link:
Bonsai Empire

So I must proceed in the dark at my own risk. :S

Never mind. I went back with cash (lots of it) in my pocket to purchase that cast iron pot but when I looked at again I decided that it was much TOO EXPENSIVE and I went back home without it. Thank you for helping me to make the right decision.

Depending on how confident you are in your chemistry and botany you could seal the inside of the cast iron pot with something safe. I have no idea what that would be. Some Google research might produce an answer.

Yes, it sounds as though it might be possible. Perhaps coating it with plastic might do the job if water doesn't eventually seep down the sides and cause it to rust anyway. But on close scrutiny, I decided it was much too expensive and I decided against purchasing it. Thank you for your advice.