There is no way to know whether this will cause damage or not right now. It's something you will have to wait and see. Any scar tissue that could pose problems won't show up for awhile. You really need to see an obgyn NOT the ER.
I had retained product after a d&c and I didnt know.. the bleeding got lighter than a week later it started up again then I started passing clots and had extreme cramping.. My HCg was around 50 when I passed the product.. and went down quickly after that.
Have they done anything else, such as give you a pill to cause contractions? I would think that if you go to your ob-gyn (not the emergency room), you could talk about getting an ultrasound-guided D&C to get the retained tissue out of there.