Lease with the option to sell. Make it sound as if it's to their benefit.
If the domain market (esp .net) continues its slide while the restaurant thrives, ask a fair price, reconcile the difference and be over with.
If the market picks up, offer apologetic news to your tenant raising the rent to the full extend allotted in the lease.
You may be able to use a precompiled, "standard" lease agreement. It would be good if you could have an attorney look it over though.