Lets say in this TL he abandons the Schlieffen plan at this point and decides to just occupy any French industrial and resource regions in reach before going over to the defensive.
(Why? perhaps he figures the remaining forces are not going to achieve a decisive victory in the west, and even with a major victory the French won't make peace with the Russians doing well in the east so a long war is likely regardless)
Instead of swinging east or west of Paris. He will advance to a line roughly from Verdun to where the Somme reaches the sea before going over the the defensive and focusing on the east. Securing this would be very doable in the time frame of August 20th. Would secure the Pas de Calais and all the major French industrial resource areas ans well as being relatively short. Securing all of Belgium might for the Belgians into an armistice.