O you who have believed, when [the adhan] is called for the prayer on the day of Jumu’ah [Friday], then proceed to the remembrance of Allah and leave trade. That is better for you, if you only knew.
al-Qurtubi says that the Madhhab of al-Shaafi'i is that any sale conducted during the time of Jumu'ah prayer will be contracted and won't be invalid.
al-Qurtubi holds the opinion that the correct opinion is that the sale and trade conducted at the time of Jumu'ah prayer is invalid and uses the following hadith to support his view:
'Aishah (May Allah be pleased with her) said:
The Messenger of Allah (sallallaahu alayhi wa allam) said, "He who does something contrary to our way will have it rejected."
Ibn al-Arabi [the Maaliki jurist] also holds the opinion that the sale and trade conducted at the time of Jumu'ah prayer will be in valid.
The sale and trade of those who are not required to attend the Jumu'ah prayer is valid.