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For starters, this forum is to help with difficulties of a technical nature, and as such does not provide for modding help. Which is what you were doing by manually editing data in a save game.

But, to answer your question. You gave yourself too much. So much, that the number the game uses to store the amount approached the maximum that can be represented. Due to the nature of processor technology, those numbers roll over into the negative equivalent when you exceed the maximum. Which is precisely what happened to you.