On Charles the Fat's Deal with Viking Warlord, Siegfried, In Which In Exchange for Concluding the Siege of Paris, the Frankish King a) Handed Over Massive Amounts of Silver and b) Allowed the Viking Contingent Free-Reign Into Burgundy Where They Could Continue Their Plundering with Scant Resistance

Yes, it sounds a trifle sleazy and cowardly BUT being that it saved Paris from being sacked again (the Vikings had had their way there several times previously) and bought some time for further negotiations (Rollo ultimately acquiring Burgundy in exchange for his allegiance to the King of West Francia; Charles the Simple by then), it actually worked out pretty good, I'd say. See, you can negotiate with barbarians (some of 'em, anyway).