My home is 82 years old and has an Iron water supply line that runs from the meter to the basement. In the basement is the main shut-off with an iron pipe that comes off the valve. They replaced the pipes that supply the house with copper connected to the iron pipe. I noticed a little water that seems to be coming from where they connected the copper pipe with the iron one. I'm concerned about the age of the iron pipe and causing more problems. What are my chances of being able to take this fitting loose, teflon tape it and getting it back together without having to call a plumber in for a bigger issue?

What are the odds of the Braves winning the World Series? Possible but who knows until it starts. As long as the cut off is there in case it does not work.

Was a dialectric union/nipple installed? It should have been.

How long ago was the outside replacement?

Yes, there is a dialectric union. The outside probably has never been replaced.

Would this union be leaking from the threads or the union joint? Let me know, tom

I believe it's leaking from the threads itself.

Who is the "they"? If it's the city just call them back out a demand a dry union. Same if the they is a plumber

I believe it's leaking from the threads itself.

You have several options,
1) you can "back bite" the galvanized pipe and tighten the threads **OR**
2 ) You can shut the water off at the meter, unscrew the union and Teflon Tape the threads and do it all over again.
Good luck, Tom

"they" are the previous owners.