Only valid for courses sold via WooCommerce.
If you want to give fixed commission ( say $100 ) for instructor on sale of every course then follow below top
Add this code in WPLMS Theme customiser or Child theme functions.php file :
add_filter('wplms_calculated_commission_base','wplms_fixed_commission',10,2);
function wplms_fixed_commission($commission,$instructor_id){
// You can also configure this based on instructor IDs
return 100;
}