Japan’s antitrust watchdog plans to warn major automaker Nissan Motor Co. for unilaterally reducing payments to over 30 subcontractors, a source familiar with the matter said Monday. Nissan allegedly underpaid the companies by some 3 billion yen ($20 million) upon delivery of their services, in violation of the subcontract law, the source said, with the Yokohama-based company admitting it was in the wrong. The Japan Fair Trade Commission plans to issue an advisory to prevent a repeat of the situation, the source said. The subcontract law prohibits companies from unilaterally deducting payments…