I just found this topic, so I'm coming into this one reaaaaally late, so sorry about that....
But.....
First off, I totally believe that the "right thing" would be to tell the property owner the WHOLE TRUTH and let him know that a bounty was being pulled from there and you witnessed it by accident.
If you were being stolen from, wouldn't you like someone to tell you the truth?!?!?!
IMO, you might as well be a thief yourself if you're not willing to help right a wrong. And those guys were wronging this farmer. Personally, I think part of our society's problem is that people are willing to watch other people get wronged and not do anything. It sure makes it easier for thieves to be thieves when this is the general attitude out there, eh?
It's up to the OP to do what he thinks is right in his heart, but for me, I'd be going back there.
I bake a mean pan of carmel rolls that would knock his socks off. But, if didn't bake,

I'd bring a basket containing a good ham, some cheddar cheese, dijon mustard and some soft bakery rolls with a 6 pack of good bottled beer and a pound of good coffee (in case the guy doesn't drink). It's the thought that counts here - so I'd want it to look like I DID put some thought into it. I'd have some kind of business card with my phone number on it, so if he doesn't warm up right on the spot, he might feel differently later after a few ham sandwiches.
I'd be APOLOGIZING ALL OVER THE PLACE for not being totally up front on the first visit. I would explain that it was an uncomfortable situation for me and I bungled it. I'd like to make up for it now so I could sleep at night.
I'd explain exactly what I saw. He's got more of a right to know what's on his property than the two guys that were robbing the property!
Flip the situation around - there is not a single one of us on this forum that wouldn't want to know if we were being stolen from!!!
Good grief - we all return class rings - and they're not even stolen! It's just plain wrong to witness theft and then "stay out of it".
Most people appreciate honesty. And up until now, it doesn't seem like the farmer has been given a shot at talking to anyone who has been 100% honest with him. No wonder he doesn't want those kind of people on his property, huh? If someone is acting sneaky, and seems to be withholding information from you, would you be wanting this kind of character on your property?
I'd be offering to take him out there with you - and if he wants something - work out some sort of deal that works out for both of you.
What's the worst thing that happens? He says "no" to you again? He buys a detector? He and his kid to go out there?
If so, better them than the other two guys, IMO.
Off my soapbox now.