It is all a negotiation. Before they sign the lease, and they ask for the right of first refusal, you can agree to give them that right if they agree to getting out if they decline. Then write in the agreement (if they agree) that in the event they do not buy it, then they have 30 or 40 days to get out. But, if this property of yours is with a lease agreement already in place that does not specify when they leave after they decline the opportunity to buy, then go with something like Michael suggests. Incentives work well.