Japan’s antitrust watchdog has probed the Japanese unit of global credit card giant Visa Inc. over alleged unfair transactions, sources familiar with the matter said Wednesday. The Visa unit is suspected of charging higher fees to credit card companies that did not use its network to check credit information, forcing them to stop using its rivals’ networks or switch to using its networks, according to the sources. The Japan Fair Trade Commission conducted an inspection at the office of Visa Worldwide Japan Co. in Tokyo as the alleged practice would violate the country’s antimonopoly law, they …