Scrum exists to serve your needs, not vice versa. Do whatever makes sense to you. Monitor how to works. Adjust.
Does it make sense to give this developer a vacation? Does your developer have some piece of trivial cleanup that they would like to do and didn't get to? Is there a particular story that the developer can get a head start on, and then set aside? We're not there, we don't know the situation. You are, make something reasonable up.
Remember, Scrum came out of the Agile movement. Go read the Agile Manifesto at http://agilemanifesto.org/. It is short, 4 bullet points. Half of them apply directly to this case, and agree that if an "official Scrum procedure" gets in your way, you should be doing what makes sense to you rather than following ritual.