The game stole 5000 gold from me

I had around 6k gold saved up and I wanted to finish building industry quicker, so I spent 4000 gold and sped it up. Then it took half the amount it said it would (?) skipped half the production time (???) and that was it, so I clicked again to finish it. It finished and I started building a port. But then, I logged off and rejoined the game again and I had only 1800 gold and the industry was still half-done. So the game stole gold from me.