What should I do with the 100 pounds I received from a customer who has not returned to collect them after two months?

If you despair of the owner of the money returning, then give it in charity on his behalf, based on the statement of Shaykh al-Islam Ibn Taymiyyah: "If the owner of money cannot be identified, it should be spent on the welfare of Muslims, according to the majority of scholars, such as Malik, Ahmad, and others." And if the money is in his possession, such as usurped goods, borrowed items, deposits, or pledges, and he has despaired of identifying their owners, then he should give them in charity on their behalf or spend them on the welfare of Muslims.

Among what has been reported regarding this is that Abdullah ibn Mas'ud bought a slave girl and entered his house to get the price. When he came out, he did not find the seller. So, he began going around to the poor and giving them the price in charity, saying, "O Allah, on behalf of the owner of the slave girl."

Ibn Taymiyyah mentioned that the majority of scholars, such as Malik, Abu Hanifa, Ahmad ibn Hanbal, and others, say that whoever possesses money whose owners are unknown should give it in charity, and this is what has been transmitted from the companions of the Messenger of Allah (peace be upon him).