You wouldn't have made the deal if you'd known they were different. You were doing them a favor by selling them what you thought was a duplicate. They've got to understand that you wouldn't have made the deal if you'd known the chips were different.

I'm sure whoever your deal is with would understand (if they're truly as "big" a Harrah's collector as you), and would not begrudge you the chip.

But then again, knowing you, you probably wouldn't keep the chip even if they insisted you keep it!

Look on the bright side - now you've got an even BIGGER challenge!