I don’t understand the need for disabling the ‘Include Contracts’ check box since it can be unselected if the contracts need to be excluded.

Since the Billing Generation screen is a FIXED type of screen, it cannot be modified via the UI Designer.

As a workaround, baseline metadata can be directly modified using the following query for making the field read-only.

update ms_item set read_only = 'Y'  where screen_name = 'BillingGeneration' and label_message_id = 'IncludeContracts'

Anyway, this is not a recommended approach, and the modifications can get overridden if the baseline screen metadata get updated because of upgrading FSM.