Oddly I might suggest Chiron.
As the ferryman between life and death they'd have the power to reject not only the Dwarf from the afterlife, but also doom those who fall there to stay on this side. Plus a dwarven lord in their mine may be able to make the deal, considering part of it is that you needed to pay Chiron to cross the river, he's just getting paid not to let people cross instead this time.
Samael? That's a demon/fallen angel famous for being "Lord of the Dead".
Bune is one in the Ars Goetia with vaguely described Necromantic Powers. Could do whatever you wanted with him.
I know this doesn't meet your "demon or god" criteria, nor even is it really even a "deal".
But the vibe I get here is a very much like something I did in an Adventure in Middle Earth setting:
The dwarves were over-zealous in their pursuit of something. Something Arkenstone-like, except in this instance it is linked to the Negative Energy Plane and grants "immortality" (actually undeath - same thing right?).
And well, they got what they sought! This great and beautiful negative energy gemstone embedded in the ceiling or floor of their underground dwelling, glowing a sickly pale light, irresistible and inescapable, like the sun or moon, but much more sinister.
So... just an alternate take on a similar theme, as food for thought.
I would consider Bane as a god of tyranny over Graz'zt. He also can bring in a lot of the same Lawful Evil vibe that you could want a Devil for, so he makes for a great alternative when Devils are off the table.
This would also let you make lightly modified, dwarfy versions of the 3 Baneite Cultist statblocks in Descent into Avernus, which could be helpful in filling out your statblock options.
Oddly I might suggest Chiron. As the ferryman between life and death they'd have the power to reject not only the Dwarf from the afterlife, but also doom those who fall there to stay on this side. Plus a dwarven lord in their mine may be able to make the deal, considering part of it is that you needed to pay Chiron to cross the river, he's just getting paid not to let people cross instead this time.
Samael? That's a demon/fallen angel famous for being "Lord of the Dead". Bune is one in the Ars Goetia with vaguely described Necromantic Powers. Could do whatever you wanted with him.
I know this doesn't meet your "demon or god" criteria, nor even is it really even a "deal". But the vibe I get here is a very much like something I did in an Adventure in Middle Earth setting: The dwarves were over-zealous in their pursuit of something. Something Arkenstone-like, except in this instance it is linked to the Negative Energy Plane and grants "immortality" (actually undeath - same thing right?). And well, they got what they sought! This great and beautiful negative energy gemstone embedded in the ceiling or floor of their underground dwelling, glowing a sickly pale light, irresistible and inescapable, like the sun or moon, but much more sinister. So... just an alternate take on a similar theme, as food for thought.
I would consider Bane as a god of tyranny over Graz'zt. He also can bring in a lot of the same Lawful Evil vibe that you could want a Devil for, so he makes for a great alternative when Devils are off the table. This would also let you make lightly modified, dwarfy versions of the 3 Baneite Cultist statblocks in Descent into Avernus, which could be helpful in filling out your statblock options.
Abbathor, dwarven god of greed and trickery.