Adding/updating a child (detail) record using URL formula

you can probably avoid that pop up question by going to advanced properties for the Estimate Header and and checkbox  "Save parent record automatically when a child record is created"

As for the button not populating, that probably happens if you have not tabbed out of the input field for the discount before clicking the button.  

You may need to contrive a way to get the user to tab out or maybe only allow Apply Discount in View mode.