So I found this issue. I've finished the game, but when my cash pool hit 932 pentalions, the cash froze. It didn't convert to diamonds, since I haven't chosen to.
My suggestion is to make cash turn into diamonds default. And when you want to upgrade something and you don't own enough money (and you have at least 1 diamond) in your cash pool, the game calculates the difference between diamond cost and upgrade cost and gives you the upgrade + gives the money difference directly in your cash pool.
Anyway, nice game, but kindda short to complete =)
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