Use this formula:

=AND(A1<"",A2<A1)

Make sure that the option for "Ignore Blank" is unchecked.

"ollie" wrote:

I am looking for a formula to place in Data Validation / Custom so that an
amount cannot be placed in cell a2 unless a1 has a value in it and the value
in a2 must not exceed the value in cell a1.