The feud is tied between Finn Balor and Baron Corbin.
Finn Balor and Raw Constable Baron Corbin have been feuding since before the Extreme Rules pay-per-view as their feud reignited soon after Corbin was appointed constable. The two of them have been consistently trading barbs on Twitter and on WWE television. Corbin was victorious one week on Raw over Balor which brought their feud to 1-1 in singles matches on WWE programming.
During Raw this week, Balor distracted Baron Corbin during his match against Roman Reigns and then Balor attacked Corbin after the match. Mike Rome announced a match between the two wrestlers in an interview which is featured in the tweet below.
SummerSlam takes place on August 19th and has multiple Championship matches scheduled including the WWE Universal Championship which Corbin has been involved in with his role as Constable.