Once word gets around, opportunities that your players might never have heard of will suddenly become available to them. Those who traffic in rare, powerful, and expensive items will target that new money, and begin to dangle things in front of them that they just can't resist.
Of course, since you know what lights up your players, you can be sure they're practically irresistable. The key from your perspective is to make these items expendable. By way of example, in a D&D-esque setting this may be an amulet of ressurection (auto-activated, 1 shot only), a potion of titan strength or some other extreme physical enhancement, a powerful wand that can't be recharged, magic arrows, etc. Even if you have to offer them something that's unbalancing, it's temporary. What's more, you can insert extraordinary circumstances into the next adventure or two that give them the opportunity to expend those resources.
The trick is to make them feel like they got their money's worth without unbalancing your campaign and without making them feel railroaded. When they perform some incredible deed with their high powered item, make sure they hear that the story has become a song that's frequently played in local taverns. Or make the player's become synonymous with overcoming impossible odds or exceeding one's abilities, e.g. overheard in a bar: "Fight them off? There were 6 of them! Who do you think I am, (insert character name here)??"
Also, if the item is rare and powerful enough, some players will just hoard it, always figuring they can find some other way out of their current situation, and they better save that special item until they REALLY need it. I've seen items like that sit in inventory until the players advanced to the point that they weren't even overpowered anymore.
Lastly, (and this is a good general technique), if the players still feel manipulated, drop hints that the manipulation might be in-game, not GM fiat. Have them learn things that unexpectedly tie the adventure where they acquired the cash to the event where the results of that cash were expended. Having shadowy manipulators is a useful plot device, and even if you don't have an immediate plan for them, they can be easily developed into future campaign material.