I sit and think a lot and understand.. It was so easy to find it...
The customers invoice was e.g. 100€ and you put as payment 105€. The software understands this and reminds you that you owe 5€ to customer..
The only think you can do is to make a payment of 5€ (that means you return to him 5€), but not applied (connected) to an invoice. Then the Balance will be zero
Soooo simple