How to calculate negative profit margin?

I sold a pair of shoes last week a \$150.00. The cost is \$100. My profit margin for last week is 33% (Revenue - Cost) / Revenue

What if the customer returned the shoes? Is my profit margin $0$ or negative? (Revenue - Cost) / Revenue?

What if I sold 3 pairs of shoes to a customer at \$150.00 each. (The cost of each is$100)... and the customer returned 1 pair. How do I calculate my profit margin?

If the person returns the shoes for a full refund, then your profit margin equation becomes $$\frac{0-100}{0}$$ Which is an undefined quantity, so I would say that if they return the shoes, there was essentially no sale, and so it doesn't really make sense to talk about profit margins. If you want to define this as $0$ (no sale implies 0 profit margin), you could probably argue that effectively.