What is the ruling on selling during prayer time, and does its ruling differ between Friday prayer and other prayers?

Congregational prayer is obligatory for free men, and it is not permissible to be preoccupied from it with selling or other activities, except out of necessity. Selling during the time of congregational prayer—if the seller or buyer is not excused, or if the completion of the sale is not necessary—is valid but sinful. However, selling at the time of the call to Jumu'ah prayer is forbidden and invalid.