Tough one because the trade was accepted. I myself, when dealing with a very lopsided trade, don't go back and forth on it. I spell it out clearly why I counter the trade as I do, and unless the other side accepts it, then the trade probably falls through. I am willing to trade without looking at the exact value from any guide but I am not willing to trade card for card when dealing in different eras. Going back and forth is cause for what has happened to you.
You could try to explain it to the other trade partner but chances are he will want the deal as it was accepted knowing that he got the better of you dollar wise.
Bruno
--------
Check my Profile page to see my 2023 Goals and my Lists of sets near completion (5 cards or less) or sets getting close (less than 100 cards missing and 75% complete).
https://www.tcdb.com/Forum.cfm/Page/B/ID/0/?MODE=VIEW&ThreadID=25745&C=0