A.T. Robertson Commentary Mark 7:29

"And he said unto her, For this saying go thy way; the demon is gone out of thy daughter." — Mark 7:29 (ASV)

For this saying (δια τουτον τον λογον). She had faith, great faith as Mt 15:28 shows, but it was her quick and bright repartee that pleased Jesus. He had missed his rest, but it was worth it to answer a call like this.