The most straightforward answer I can give you is: have them gain more XP than wealth during the next encounters, until you reach the intended balance. In games where XP and money (or their equivalents) are independent, it's fairly easy. The PCs are now over-equipped, so they should be able to handle some higher level fights with bigger XP awards. Choose enemies that work fine with close to no equipment. If necessary, add some XP bonuses to help level catch up to wealth. Keep in mind that the PCs will level up faster than normal while you are fixing the balance. Be careful if this causes pacing problems for your campaign or your players. Drop some items now and then, just to keep your players content. Getting treasure is fun.