In 1658 at the Battle of the Dunes the Commonwealth of England captured and Garrisoned the Port of Dunkirk, keeping it in the eventual peace treaty that ended the Anglo-Spanish War. A few years later, after the restoration, Charles II sold it to the French for (and I got the number off of Wiki) £320,000.
But what if the returned monarch decides to keep his toe-hold on the Continent? Does Dunkirk become a commonly traded port, taken and lost in the series of Anglo-French Wars that would occur in later years?