The prophet was a gravedigger before. And he has nice clothing. He would like to become a prophet.
Say that you are a prophet and that you two can switch jobs.
First you have to show that you are a real gravedigger by digging your own grave.
Easy. After you have talked with the gravedigger you can take the spade and just dig from the garbage.
Now go talk to him. He is ready to be a prophet. He just needs propert attire. Give him the sheet from the casket.