What's the best way to handle discount codes in customer service?

As an e-commerce manager, I'm struggling with managing discount codes during customer service interactions. We frequently encounter situations where customers claim they didn't receive a promised discount, want to apply codes retroactively, or argue about expired promotions. I need a comprehensive strategy that empowers our customer service team to handle discount-related inquiries professionally while protecting our business's financial interests and maintaining customer satisfaction. What are the best practices for creating clear policies and training our team to manage discount code interactions effectively?