One such fascinating concept sees much use in a myriad of systems: the obolus (plural: oboloi).
The obolus is a special kind of coin used to grant payment for passage of the dead over the River Styx. It was a hard good that could not be burned, often places in the mouth of the deceased. It insured that Charon would ferry the dead across the first river they encountered in the underworld, and its become something of a standard of interrment ever since.
You are, payment for the dead would in this context suggest that currency acts as a kind of agreed-upon means of barter in the Underworld. It is a symbol and something of value, be it for its perceived value or symbolic worth, and corresponds to the axiom “as above, so below.”
It is, by extension, logical that this currency would be of import to a number of beings who would call the other world their home. It’s an offering and means of transit, perhaps even a means of transport outside of one’s station. So where blessings and adjurations offer no persuasive power, consider the offering of coins and other physical goods.
It is a simple, yet powerful practice. ;)