Contacting a store after paying less for an item due to an error in their system

Q: I paid less than I should have when I went to the supermarket a few months ago due to an error in their checkout system. I messaged their customer service online on the day and they said “Thank you for your honesty. There's nothing much we can do about it at here at Customer Services but if you do wish to get rid of the difference you can return to the store and speak with the Customer Service desk.”

Am I sinful if I don’t go back and return the difference? I’m not 100% sure if I still have the receipt.

Making statements against the weather

Q: I say bad things about the rains and winter that I don't like winter season and why is it raining now. Sometimes I say that I don't like this vegetable. Is this kufr?

After watching videos regarding kufr I struggle with everything that it might be kufr.

I met an Aalim and he told me that one with pure intentions speaks kufr and affirms on it is a kaaffir so don't worry about what is done unintentionally.

Giving zakaat to one's son in law after one's zakaat date

Q: Can I give zakaat money to my son in law who wants to study... I give my zakaat every year in Ramadhaan and he will only start studying around June... Is it ok for me to keep the amount aside up until June? Is it ok that I do not disclose to him that it is Zakaat money? He does work and earns a low salary which goes into necessities and he does not have the nisaab amount.