Can I give the store an unwanted fish for them to sell?

Yes they will take your fish as a donation and sell it to someone else. You can always surrender your reptiles and fish to them and your snake food too

I was there last week, and I asked them will they take the rat's back in a few days if my snake doesn't eat them. They said they could take them back as a donation, but not give my money back.

yes they will in some quarantine tanks on the side, but what do u have?

I don't think so