My wife wants to donate her frozen milk to a random stranger on Facebook, but I'm refusing.
I'm not sure I understand. Why don't you want her to donate this milk?
Because that milk is for our child. She's being selfish.
I would think that your wife would still be able to produce milk for your child even if she donates some to a stranger. It sounds like she's doing a nice thing for someone. Obviously, if this would interfere with raising your child then you should try to talk to her about it.
You can tell her whatever you want pal. I'm sticking with my point.
I think you should let your wife donate this milk. If she has enough to keep your child healthy, then you should let her help out this stranger. It's good to help other people out, and if you try to stop her, she will just feel annoyed and frustrated with you.