Heating Company that installs and services boilers. $100 to $300 depending on if you have isolation valves or they have to drain the whole thing.

That is a REALLY BAD solder joint.

The problem is that the entire system is likely to have to be drained to get that line water fre.

Them some pipe replaced, then a refill, then running the system, then bleeding the air released from the water as it is heated, then some more water added, repeat until there is no air (often over a couple days).

If you can at least perform the repeated bleeding and the stem has auto fill setup it will be less than repeated technician visits.
It is a small repair with a lot ofafter the fact activity.

While there appear to be some valves to isolate, they do not look extensive and simply moving the older valve with the square fitting may cause IT to start leaking (if it can even be moved or seal off).

You should not have copper lines joined directly to steel lines as it appears you have

I appreciate the help, guys. Adding water and bleeding air shouldn't be an issue for me. I had an HVAC company come out and check up on the boiler about a month ago, and while it wasn't wet at the time, they didn't say anything about the crappy joint or any evidence of leakage. This is the same HVAC company that bought out the people who installed it originally.

brickeyee, what steel lines are you talking about here? I thought I was looking at copper and cast iron.

I'm not an expert but I agree that the evidence of leaking looks like it's more than what could have happened in the last month. They could be playing dumb (they could be liable for bad work of prior company), or acting that way because that's how they are. Either way, their failure to bring this to your attention or fix it negates the value of paying them to come out.

I'd call another company.