I don't see this as money vs ethics as you obtained the domain legally and without malice. So keep the domain and develop it and / or sell it
Earlier this year I sold my Alphaflash.com domain for $xx,xxx
The domain had reached the final stage of the expiration process without my knowledge and was a week away from going up for public availability. I received a call at home from a woman with an accent who said she was interested in buying the domain. She said she'd been trying to contact me for months but my domain info was incorrect and this was her last effort to contact me. I thought that was odd.
I looked at my domain and realized it was expired so quickly spent the best $6.95 of my life to renew it. After doing a bit of research I found that
this company was registering Alpha Flash as a service mark/trade mark in many countries with one pending in the U.S. The negotiation process was a few months but ended very well for me.
I bet many millions of dollars are lost every year by domain owners losing domains that others would pay them well for. It's just part of the business , not unethical IMO.