Insert value into cell if other cell value changes

[kriscons]

Hi

I need a solution to do the following please:

Cell A1 contains a Total Invoice Cost value (e.g. 9.99)
Cell A2 contains a Total Paid Value (empty by default)
Cell A3 contains a list menu with different options (Paid, Credited, Overdue etc).

When Cell A3 changes to 'Paid' I want Cell A2 to automatically update to the value of Cell A1 unless it is otherwise overwritten. If the value of Cell A3 is not 'Paid' then the value of Cell A2 remains empty.

Can anyone help me with this bit of logic?

Thank you in advance.

[dosydos]

=IF(A3="Paid",A1,"")