Outstanding balances negative

Hello,

I am new in Booking.com listings and ending my first month.  I usually get the payment without commissions, as it should be. But now I see that Outstanding balances are negative ( Finances > Documents and invoices ) why is that? I am trying to understand it, but as new user I am having issues to understand it. Could any of you explain me a bit why it is negative if I already paid for those comissions? 

Thank you.