Do Catholics take an oath never to buy from a Protestant what they can purchase from a fellow Catholic?

The Catholic Answers

No. If a Catholic takes an oath injurious to an enemy because he is an enemy he commits a serious sin. Catholics are quite free to deal with whom they please in business. The ordinary Catechism puts the question, “Are we obliged to love our enemies?” and gives the reply, “We are obliged to love our enemies. Love your enemies; do good to them that hate you; bless them that curse you; and pray for them that persecute and calumniate you.”

Radio Replies Volume 1 by Rev. Dr. Leslie Rumble MSC and Rev. Charles Mortimer Carty