Using PostbackURL and AJAX

I have an ASP.Net/AJAX page named ViewLoads that is a fairly complex collection of updatepanels, multiviews, and controls of all kinds. It's working great.

Now I have added a control (cmdExportCampusTotals) which really has no code but has PostBackURL set to ~/GenerateFile.aspx. This activates that page in a hidden iFrame on the original page and downloads an Excel spreadsheet using this VB code:

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        With Response
            .ClearContent()
            Dim filename As String
            filename = PreviousPage.SharedExportName
            .AddHeader("content-disposition", "attachment; filename=" & filename & ".xls")
            .ContentType = "application/ms-excel"
            .Write(PreviousPage.SharedOutput())
            .End()
        End With
    End Sub

Everything is working fine, and the original user interface still displays unscathed. But now, clicking any control in the page causes a postback to GenerateFile.aspx. How can I reset the page so that it posts back to ViewLoads.aspx instead?

Some of my searching has suggested using the RegisterStartupScript of the ScriptManager control, but I have not been successful in making that happen.