How do I handle refunds on discounted items?

As an e-commerce business owner, I'm struggling with managing refunds for items that were originally purchased with a discount. It's challenging to determine the refund amount - should it be the original price, the discounted price, or something in between? I want to create a fair policy that protects my business's revenue while maintaining customer satisfaction. What are the best practices for handling refunds on discounted merchandise, and how can I create a clear, consistent approach that works across different types of promotions?