Trade-in credit never received

I did trade-in a year ago for 2 phones and added a 5th line on the same day last Sep. Still fighting for a credit. The service rep who took the upgrade order told me that I'll get $1000 per phone which was the offer at that time. I accepted instead of waiting for a thanksgiving deal. On the same night I cancelled the 5th line.  I didn't get an email with the upgrade and ATT doesn't send one as of last sept which gives sales reps lot of appetite to give false promises to book upgrades on their names. By the time, order was under processing so as soon as I received it, phone was returned. After 2 weeks trade-in phones were also returned. The ATT back office folks who opened one of the trade-in phone was put under the 5th line as the phone was received, so ATT keep saying that one of the phone was not received, despite of emails we received from ATT that all three phones were received. Trade-in credit for one phone has been getting for only $350 ( not a $1000 as promised) and no credit is given for the 2nd phone until today. I've been calling them. When I called yesterday, Brain a senior manager from Loyalty dept (I have been with ATT for the last 16 years), said, since I cancelled the 5th line, is the reason I did not get a trade-in credit and when asked why I'm getting credit for the other phone, he said I'm lucky, and it was a mistake and he doesn't want to take it away. Then I asked, what if I didn't add 5th line on that day, did I get a trade-in credit, he said he don't want to answer hypothetical question. I told him I'll contact my attorney.