|You can't use have a formula in B1 that is dependent on A1 (such as an IF statement) and also allow the user to enter "any value". |
Once the user enters the value the formula will be overwritten.
To accomplish your goal, you can use a macro that will clear B1 when A1 is changed to FALSE and then prevent user input.
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$A$1" Then
If Target = False Then
.Add Type:=xlValidateCustom, _