In Asp.net Web application Retrieve Gridview footer dynamically

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In Asp.net Web application i have a problem to Retrieve Gridview footer value.

I want to retrieve grid view footer text. that footer contain net amount ,this net amount changing Dynamically,
if retrieving the footer value only get older value not get new changed value .

My code is

<asp:TemplateField HeaderText="Amount" >
                                       <ItemTemplate>
                                           <asp:Label ID="lblAmount" runat="server"  Text='<%# Eval("Amount") %>'></asp:Label>
                                       </ItemTemplate>

                                           <FooterTemplate>
                                               <asp:Label ID="lblTotal" runat="server" ></asp:Label>
                                           </FooterTemplate>

                                   </asp:TemplateField>

Label lblTotal = ((Label)gvDetail.FooterRow.FindControl("lblTotal"));
        NetAmount= Convert.ToDecimal(lblTotal.Text);

script portion to change footer text

NetAmount = parseFloat(NetAmount) + parseFloat(Total);

var ctrlNetAmount = "ContentPlaceHolder1_gvDetail_lblTotal";
                    document.getElementById(ctrlNetAmount).innerHTML = NetAmount;

After changing the footer by script igot thet value in footer an currectly display it. but when retrieve only get the older text of lblTotal,ie older value

pls help me