The dead man's brother - Roger Zelazny

THE CORPSE UNDER THE CHAGALL

Once an art smuggler, now a respectable art dealer, Ovid Wiley awoke to find his former partner stabbed to death on his gallery floor. That was strange enough - but when a CIA agent showed up to spring him from NYPD custody, things got a lot stranger.

Now the CIA is offering to clear up the murder charge, but only in return for a favor: They want Ovid to fly to Vatican City and trace the trail of a renegade priest who has gone missing with millions in church funds.
What's the connection?
The priest's lover, a woman Ovid knew in his smuggling days.