What should I do with the amount that the ATM mistakenly dispensed to me from another person's account, and which the bank was unable to identify its owner?

If you wish to keep the money until it is confirmed that its owner cannot be found, perhaps he will be found in the coming days. If you wish, you may give it in charity on behalf of its owner, on the condition that if he is found, you give him the choice between approving the charity, in which case the reward is for him, or taking the money from you, in which case the reward is for you.