In the aspx of the child page (head), put this
<script type ="text/javascript" language="javascript" >
function callParentClick() {
window.opener.document.getElementById('<%=btnSave.ClientID%>').click();
}
</script>
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In the button click event of the close button(code behind .cs file) put this
ClientScript.RegisterStartupScript(this.GetType(), "CallParent", "callParentClick();", true);
//parent page
private void SavedataButton_OnClick()
{
Session["Data"] = ....
}
//popup page
private void CloseButton_OnClick()
{
object data = Session["Data"];
}