My parents are not observant, and they unsolicitedly deposited a check into my bank account as a gift for me on Shabbos. My question is if I am allowed to benefit from this money, or am I obligated to return the money?
You may use the money but you should wait one extra business day after the check clears. The reason is that by your parent depositing the check, they essentially were telling the bank, who are essentially gentiles to put that money I your account. Therefore it is considered as if a gentile did melacha for you, therefore you should wait b’kday sheya’asu, so you will not be benefiting from what the gentile did for you.