Short story. The Butter Spirit’s Tithe first appeared in Emerald Magic, edited by Andrew M. Greeley; Tor Books, 2004, and was most recently reprinted in Muse and Reverie, Tor Books, 2009.
Conn O'Neill is trying to make a go of it as a Celtic guitarist until he inadvertently runs afoul of a vengeful spirit. Cursed for seven years, at the end of which his soul is promised as a tithe to the fearsome Grey Man, Conn's only hope lies with his musical partner Miki Greer. Miki knows a thing or two about Irish spirits, having lost her brother to them in de Lint's novel Forests of the Heart. She's not about to let them claim someone else.