Mistakenly receiving extra money into one's account

Q: I'm using a banking app, so I have transferred funds from 1 account to another but the transaction failed and the balance has been deducted from my account. After a few minutes, I received a refund but my 2nd account has also been credited with that amount... So I'm asking that should I return it to the merchant as I have been paid this extra amount mistakenly.

Second husband being a mahram for the wife's daughter

Q: I am getting married to a woman who already has a daughter, from a previous marriage. Would I be a Mahram for this girl, because Allah says:

وربائبكم اللاتي في حجوركم من نسائكم اللاتي دخلتم بهن فإن لم تكونوا دخلتم بهن فلا جناح عليكم

Would it be ok for me to travel with this girl?