I'm using Windows 10.
I'm in Wahoo Beach, in the Jungle Farm Plot.
1. Click on the mailbox and click on "Send Letter".
2. Click on your username in the Player List.
3. In the Gold textbox, insert the quantity you have.
4. Click on "Send".
5. You received a new mail, don't check it.
6. Click on "Send Letter" again.
7. In the Gold textbox insert the quantity you want to have in negative numbers (for example, -1000000).
8. Click on "Send".
9. You received a new mail. Again, ignore it.
10. Enjoy your new life 8)