Sanders needs the debate far more than he does, so he has enormous leverage. Vet the questions ahead of time, format it in the usual forum style where the actual direct interaction between the participants is minimized, emphasize things he and Sanders have common ground on, like trade and--before Hillary forced Sanders to shift leftward in primaries--gun rights and immigration skepticism, be disciplined enough not to launch a verbal kill shot at Sanders, and otherwise use it as an extemporaneous stump speech opportunity where he is at his best.
The downside risk is easily mitigated. With his charisma and frame control he's masterful at avoiding the body blow. This isn't even a debate he wants to decisively 'win'. The overarching objective is to prop Sanders up to make Hillary's nomination as unsatisfying to Sanders' supporters as possible.
Lock Hillary out of a debate with the largest audience in American history--her elective absence being something both he and Sanders could allude to in their opening remarks. Endear himself as much as possible to disaffected Sanders' supporters with an eye towards the general election. Accentuate Hillary's ire towards Sanders to diminish the salience and status of the position Sanders is offered in a Hillary administration as much as possible. Sanders as Hillary's VP is the worst case scenario and anything that makes that outcome less likely is worthwhile. Get Hillary to offer him something lame like housing secretary instead.
That said, net-net he still benefits from having created this speculation before reneging. The expectations for Hillary to debate Sanders again are obviously higher than for him to have done so. Everyone looks apprehensive about going one-on-one with Sanders, and an enhancement in Sanders' image hurts Hillary, with anything hurting Hillary helping him. Sanders ends up garnering more attention over the course of the week than Hillary does. He avoids the potential debate ambuscade.