What do you mean by “sound”? Why doesn’t David Chaum’s eCash from 1982 [1] qualify? That’s an on-line system. In 1990 he, Fiat, and Nairobi proposed an off-line system [2].
The latter works fine if you're OK with completely trusting the banks. Local quantum computation gives you provably sound, decentralised, unprintable digital cash. Nice paper though, I wonder if you can make it quantum resistant.
[1] http://www.hit.bme.hu/~buttyan/courses/BMEVIHIM219/2009/Chau...
[2] http://blog.koehntopp.de/uploads/chaum_fiat_naor_ecash.pdf