It is testing, that's why I stopped doing it ;-)

There is a fourth option for traveling in the future. You are the same person in the future, so you could just take the money back with you when traveling back.

I think I solved it, it's quite simple.

When traveling through time you're actually causing a hardfork. When traveling back you're going back to the original fork.
So whatever happens in the second fork in the future is not guaranteed in the fork you're traveling back into.