The second their internal polls show Fritz down outside of the margin of error (I am guessing he is down but within already), they will agree to the debate.
And it will be on Sunday.
It would be a high-stakes gamble. Their only hope is that Mondale wouldn't do as badly in the debate as the damage from refusing to debate at all.
Maybe they hope to pump him up with enough one-liners in the next two days so that he appears to be sharp. Tough challenge.