For special situations where you want to setup an offer that ignores maximum total discount option in the system, such as ultra last minute discount, Booking Manager allows you the option to do just that in the following way
In the Preferences > Reservation Settings > Invoice, there should be a maximum total discount option set. Please check if this option matches your company conditions. Usually companies set this to a maximum of 15 to 20 percent.
In Setup > Discount Editor you are able to create a new Discount/offer and set a special rule to ignore the maximum total discount. In addition you will also be able to exclude other discounts while this offer is valid.