This trade appears in 2 trade trees — the full chains are traced there:
Reported August 19, 1993/20 (Wikipedia cites Aug 19; PFR lists Aug 20) -- confirmed as a trade, not a release-and-signing. The Vikings dealt Rich Gannon, benched for Sean Salisbury late in 1992, to Washington for a conditional 1994 fifth-round pick (No. 134, DB Shelly Hammonds). Gannon opened 1993 third-string behind Mark Rypien and Cary Conklin and started four games (Weeks 12-15) for a 4-12 Redskins team.
On August 20, 1993, Minnesota Vikings sent QB Rich Gannon to Washington Redskins for a 1994 fifth-round pick (No. 134, became Shelly Hammonds).