The reason there is a minus figure for Frank is because you have recorded expenses payments to Frank, but have not recorded what bills he paid on behalf of the company, or mileage/subsistence owed from the Company to Frank. In essence, the minus figure is telling you that you have overpaid Frank expenses, or that you have not recorded the company expenses that Frank personally paid.
If you click on Wages > Employees > Frank > Expenses Statement. Change the “From:” date to “1 Dec, 2013” and click update. On the report, you’ll see that the company has paid Frank at total of €1,825 in expenses. There’s nothing in the amount column, meaning nothing that Frank has paid for (or mileage/subsistence expenses) have been recorded.
Bullet won’t force you to do anything – it’s just showing you the minus figure to let you know.