What if Na Nach really means Please Rest (in Peace Nachman)?
That would make THIS the "promise" given before the death of Josef: "pakod pakodeti", the code word given to the elders by Moses. That would mean this was a commandment accepted by all before the giving of the Torah!
What could have come before that if not the oath of Jacob?
The Maaser. Is conditional...do you have bread and cloths? If yes, you need to give Maaser.
Page 70 in the Gemarra (vrs 70 that gave the pakod pakodeti oath to Josef) mentions Maaser.
Why did Rabbi Nachman say "I could bring the Mesiach, but rather have 10 of you that are real talmidim". why did Josef send the ten to pick up Benjamin and made sure they would carry the gold back and forth on donkeys using many shoes on the way from Canaan to Egypt (while their children were starving for grain, they were feeding tired donkeys with the little they had).
You should read that page 70 if you care about the Shema Israel you say twice a day.
I believe that anyone scrupulous about this Halacha will be able to know who Saba really is.