I agree about returning the full amount. but I do things a little differently that may not work for others.
if my doctor is contracted, I return any amount that is over the contract payment amount. If my doctor is not contracted, I return the amount over the carrier's usual and customary fee.
Any money I return is sent with an accompaning letter describing the amount being returned.
If the insurance company says Medicare is primary, I demand they provide me with verifiable proof. I also inform them they cannot offset or take back any funds without our written permission (This is in our state refund law).
Now, if they just want their money back for a fivolous reason, i write to the CEO, I inform him, we have outstanding claims and I am more than happy to do as they do by offsetting an unpaid claim against their request. Yes, this can be a numbers nightmare but at least I keep the money instead of giving it back and begging for the money for the unpaid claim. I did check with my State Insurance Commissioner about doing this and he said there is nothing in our state law that says we cant do this as long as I give the insurance company the same ability to say no like we can. Some agree and some dont but at least I gave it a try.