Are you referring to a standard field or a custom one? Is it been made READ-ONLY at the layout level? If its a custom field, what you can probably do is to NOT make it READ ONLY at the layout level. Instead, have a validation rule written on it with the criteria that it should not allow for any values unless the Campaign Status=Approved. In addition, you may also want to add a Help Text on this field saying that this field should only be populated once the Campaign Status is Approved.
Are you referring to a standard field or a custom one? Is it been made READ-ONLY at the layout level? If its a custom field, what you can probably do is to NOT make it READ ONLY at the layout level. Instead, have a validation rule written on it with the criteria that it should not allow for any values unless the Campaign Status=Approved. In addition, you may also want to add a Help Text on this field saying that this field should only be populated once the Campaign Status is Approved.
Let me know how it goes!