Responses To The "Save The Date" Ca

My daughter is getting married in June so in January she sent out "Save the Date" notices to everyone on the guest list. After receiving the cards, a few people
Have called to let us know they won`t be able to attend because of other commitments. My question is should these people still receive an invitation? I think not, because we know they aren`t available and don`t want them to feel like they need to send a gift. What is the proper way to handle this?