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I think it's a bug since I did get it when playing and noticed it. If you wait from time to time you're able to even buy stuff with unlimited funds, but it also goes negative and in those moments you can't buy anything. 

I think it's because you should try not to fertilize past when the tree looks really huge and you see a rate of 146 or higher at the top to avoid this problem.  I played it again recently and included some screenshot somewhere of how the tree looks when you shouldn't upgrade if you want to avoid getting the negative money.