Unfortunately this is a bug that Storm8 knows about but there hasn't been a fix. Their suggestion is to keep your coins below the maximum number.
per Elsa at Storm8: 2,147,483,647 is the maximum integer in 32-bit. Any value higher than this can cause issues or present itself as an error. In the case of the game, it will appear as a negative number. Spending Coins to be lower than this number will resolve it.
The game does support higher values than this, but if the device hardware/software is also 32-bit, then this can still occur.