Britain was more likely to agree to peace and a possible alliance if Hitler was able to sweeten the offer with the return of thousands of troops captured at Dunkirk.

Hitler was not trying to let the Brits escape due to some delusion about the Brits making an alliance with Germany. Those British troops weren't on the continent because they were on holiday. The Brits barely avoided a complete disaster at Dunkirk. It wasn't because Hitler decided to be Mr. Nice Guy for a couple of weeks.