They return with the money (and expecting a double-cross) only to find the guy dead and his place torn up. Then the city guard arrives...
You could have the employer be totally honest and pay the party as promised. Later on the PC's can find out that the money they brought back didn't exactly belong to thier employer and the real owners would like a word with all those involved with its removal.
So because the players figured out your plot you want to screw them?
Double-cross them as planned and then start of making different plots.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.