Do I incur sin or expiation after swearing on the Qur’an that I did not take money, knowing that I had returned the money before swearing?

Your oath not to take money from your friend's pocket, while you knew you had taken it, is considered a yamin ghamus (a deeply sinful oath). As long as you have returned the money, you must repent and seek forgiveness. According to the majority of scholars, you are not required to offer expiation (kaffarah), though it is preferable if you do.