What to do when the double-cross is expected?

[Ruslanchik]

Oh yeah - have them jumped when they get back to the dude's place.

Just not by him

If he's a bit on the edge, he's bound to have offended a few other adventuring groups. So, another band have taken him prisoner (he's bound and gagged and tied to a chair, Reservoir Dogs style), and this mob are waiting for the PC's to turn up and take the loot - which they regard as theirs as part payment for a previous whacking.

Once the PC's spot their employee in a bind, they have to decide whether to rescue him or double-cross him by splitting the loot with the other gang. And if the new lot get away with some or all of the loot, the Face (when untied) can re-employ the troop to go kick heads (with a promise of more cash in return).

This. Absolutely.

Although, you should make the other group's motives for attacking unclear. They could be good guys bringing justice, or bad guys seeking revenge. Either way the party will have to decide quickly whether to defend the untrustworthy guy (a known quantity), strike or deal with the newbies, or high-tail it.