@Nikki404 ask them to send an inquiry (not a request to book) for their new dates. You will then have the option to respond with a "special offer" on which you can set a minimal price.
If you want to allow them to stay completely for free you probably can't do that within Airbnb because Airbnb is always going to charge their service fees.
Another option is to go to their lapsed reservation itinerary and select "send or request money" to get to the resolution center, where you can send them a refund of any amount up to their total rental payment. However again Airbnb will not refund their service fees.