Suppose: Two different players have blue saloons, I have no money, and the roll causes me to collect one chip for a blue mine. Who do I give the chip to?
If I had designed the game, I would require the second saloon to be a different color from the first, but there are two of each color of marker, so it must be allowed.
Well, if you've worked your way into a situation where you have no money and have a blue mine that pays off $1, you might want to rethink your strategy for the next game. Seriously, I don't have the answer, but I'd probably just say "Owe you a buck" to the unfortunate saloon owner that you didn't pay.