Saw this asked on the Stardew Valley sub on Reddit and am now wondering - what DOES happen when you go pasy 99,999,999 Gold?
Most likely you break the counter. The counter goes all wibbly wobbly if you sell too much at night, so I can only guess the same thing happens if you make too much gold. Here's what I mean about the counter going wibble wobbly: Just skip to around 3 minutes to see it (though I highly recommend watching the video, DangerouslyFunny is brilliant!) But I don't know for sure if this is what happens to your gold if you max it out, don't think I've ever seen that.