At the time of Delta canceling the ticket, the OP had already canceled his other trip that he booked with miles. So there was no duplicate booking when DL pulled the plug on the 'mistake' fare or whatever you want to call it.

Sorry, but whacking the ticket 6 weeks after booking is bs. There should be some accountability and recourse for the OP from Delta. If they won't honor the original flight then they should offer to reinstate his award booking that he had and canceled at the original mileage rate. Its absurd to be blaming the OP for booking something that was available on the website and I would argue not completely obvious that it was a mistake. Its not like it was one of those $50 fares or something.

I would not let this one drop without some compensation. Whats next, to say its ok for airlines to come back and look for extra money after you booked because they raised the fares? 'Oh you booked a cheap V fare, but we are selling B fares now because we underestimated demand, so you owe us more money.' If you go to a store and buy something they don't come back later asking for it back or for you to pay more.