I am looking into donating all profits from sales made during a specific timeframe to a specific cause. But, I need to determine how to handle returns.

Would it be unethical of me to tell buyers that during this timeframe, I can only accept exchanges?

If I sell an item for $40 that cost me $20, then $20 is donated.
When the item is returned, I am theoretically -$40 until someone else buys the item.

Has anyone else donated their profits? How did you handle returns?