I wonder if you may have misremembered parts of the story. I have an event in mind where the details don't really match up with your recollection, but overall seems to bear a striking similarity.
In 1943, Montgomery and Eisenhower made a 5 pound bet that the war would be over by Christmas 1944. It was actually written down on a sheet of paper, and signed by the two generals:
Agreement entered into, Oct 11, 1943, between Generals Eisenhower and Montgomery Amount £5 - General E bets war with Germany will end before Xmas 1944. Local Time.
This sheet of paper was taken by a widow of one of Eisenhower's staff, who framed it and displayed it in her home for decades. After her death, it was sold for an estimated $20,000 at auction owing to its historical significance.
The bet's significance to Market Garden is that prior to the battle, there was a feeling of optimism and overconfidence in the Allied command. Had the operation succeeded, it was envisioned that the Allies would thrust rapidly across the Rhine and into Germany before the end of the year - which some hoped would mean the end of the war, with Eisenhower winning the bet.
Market Garden's failure put an end to that (before the Battle of the Bulge buried it).