The match was originally scheduled for Saturday, but was abandoned after 60 minutes due to a waterlogged pitch.
Coton Sport took the lead just past the half-hour mark when Ewangue Mbongo hit a low shot past the Ahly keeper.
However, veteran midfielder Mohamed Aboutrika netted on the stroke of half-time to hand his side the advantage going into the second leg, though they will go into the match with coach Mohamed Youssef in the stands following his dismissal for dissent.