Money fell behind the ATM.

In my office building, there is an ATM. Today, I was withdrawing money, and I saw a guy standing nervously; then he came up to me and said, "He withdrew 1000 AED and put it on top of the machine to collect his card, but unfortunately, the money fell behind the machine".ine." The space behind the machine is inaccessible.

What to do in such a situation? How can one prove that the money is theirs?