Is it permissible to donate 300 Saudi Riyals found at an ATM with a receipt containing the owner's information, after attempting to find the owner by advertising at the ATM for a week? And is this amount considered a small sum of money?

Do not donate the money until one year has passed since its identification. If the year passes and its owner does not appear, you may donate it on his behalf, guaranteeing it to him, and give him the choice, when you meet him, between ratifying the charity or returning the right to him. The definition of a small amount (yaseer) in the case of a found item (luqatah) is determined by common practice (urf). What its owner would usually seek is not considered small, and the mentioned amount is something its owner would seek.