tattoo_mumma - a friend of mine is on single & they haven't actually started processing the single payment supplements yet (from what they told her on the phone), so you shouldn't have recieved it yet. Are you registered for c'link online? If you are if you are getting the payment in the next 2 days, it will show on your future payments list. If its already been paid, it'll be in past payments. If there's nothing on either list, it hasn't been paid to you yet

Ella - My DH had a $450 debt a few years ago & it automatically came out of his next refund from what I remember. So if this debt is in your name, & you paid $520 in tax, I assume they'll take that & your debt will go down to the difference.
If you only underestimated by $1000, it shouldn't make much difference to your supplement, but what ever you were over paid will come out of the supplement payment from my understanding. I can't imagine it would be very much at all though hun. (BG - correct me if I'm wrong )