As far as I know, create procedure permissions can be granted (and denied) only on a database level and not on schema level. You could grant it to the user on the database level and create a trigger on CREATE PROCEDURE. In the trigger you can check on which schema the procedure was created and rollback the operation if it isn’t on the correct schema. If you choose to it this way, make sure that you test the trigger, because you’ll might block other logins from creating a procedure.

Adi