If I thought I lost a Comcast modem and therefore was charged for it, and then I found it, what should I do with it?

I ask this here because I hate Comcast, hate dealing with them, and am trying to avoid that at any cost.

But if I take it into the store, will they then reimburse me for what I was charged for it (I think somewhere around 40 or 50 bucks?) or can I sell it on Craigslist? Or is it not useable by anyone else?