It sounds very much like a scam. Not saying it is, but it sounds like it. Here's why:
> 4 months later company phones and wants their fee.
1) They phoned? They didn't send a bill in the post? Or even email it? They didn't send a bill at all?
2) Four months? They haven't thought to ask before now?
3) They haven't supplied a single shred of evidence of who they got a refund from or how much?
4) (Given what chrissa wrote, they may not be able to do the following - so I've put it fourth). They didn't do the obvious thing of deducting their fee before paying you the rest, instead giving themselves the onerous task of chasing you for it months later?
> Signed a form with the company of agreement to act on my behalf
I would go back to the form and read it very carefully to see how well what they said would happen (their contract with you) matches what's happening now. For example, if the contract says they will provide evidence, they will deduct their fees before paying you and they will send you a bill (stamped paid) as evidence, and none of that is happening now, then you'd need to be very suspicious.