Disable Yes button on Check Availability form

I created a Boolean field (Allow Negative Stock) on the User Setup table if the field isnt checked then wen a user creates a sales order and the quantity is less than the inventory levels currently Navision pops up a Check Availabilty screen. i want to disable the Yes button on this screen if the (Allow Negative Stock) field is unchecked and enable the button if the (Allow Negative Stock) field is checked