I found a way to make infinite money which only requires 12 zorkmids and 25 Firekeppers favor to kickstart (probably works with other cults too):
12 crossbow bolts from Forging Destiny = -12 zorkmids
enchant all 12 with fire = -25 favor (Firekeepers)
sell 2 enflamed crossbow bolts to Firekeepers = +30 favor
sell 10 enflamed crossbow bolts to Champions of Peace = +110 zorkmids
sum: +98 zorkmids, +5 favor with Firekeepers
Doesn't seem like the intended way to pay off the debt though.