The CVoidPayment Header object does not allow the post date reversal to be set.  Use an IBBPropBag object to void the payment.

Example:

Private Sub VoidARPayment()

    Dim oVoid As CVoidPaymentHeader
    Dim oBag As IBBPropBag
   
    Set oVoid = New CVoidPaymentHeader
    oVoid.Init FE_Application.SessionContext
   
    Set oBag = oVoid
    oBag.Property("POSTDATE", "01/01/2009") = "01/01/2009"
    Set oBag = Nothing
   
    oVoid.VoidPaymentHeader 175, False
   
    oVoid.CloseDown
    Set oVoid = Nothing

End Sub