To change a Partner's commission value referring to a Booking, there are two possibilities:

During the manual creation of a Booking

In the process of creation of Stays.net Booking, in the [Conditions] step you will be able to define the partner through the [Reservation Type > +Relations] field and, if necessary, change the commission value.
You will be able to define the fixed value (e.g: R$ 100.00) or a percentage on the daily rates.

Bookings already created previously

For Bookings already created on your Central, the alternative is to access the Booking's Page and, in the area with Partner's information, click on the little pen icon next to the word [Commission].
You will be able to define the fixed value (e.g: R$ 100.00) or a percentage on the daily rates.

For bookings imported from Sales Channel where it already informs us the value of the commission, Stays.net will not allow to make this action so that information is not misaligned between platforms.

Now that you already know how to adjust the commission of a partner by booking, how about checking out more related subjects?