Old B&W movie where the conmen get conned

A while back I saw an old black & white movie (probably from the 1930's or 1940's) where a woman got conned by some people. I think it was set either in South Asia or South America and the woman who got conned into buying some land that was supposed to have a treasure (gems or gold). Another guy (a well known actor, like Humphrey Bogart or James Cagney) helped her out by making the conmen think that the land did actually contain the gems or gold that they had told her about.

Beat the Devil

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No, that's not the one. There was a boat, but the movie was set in a heavily forested location. I think they tricked the conmen by getting some real gold or gems and either pretending like they found it on the land or by planting it there so the conmen would think that the land was actually valuable.