Hi akhter,

Check this example. Now please take its reference and correct your code.

HTML

<style type="text/css">
    body
    {
        font-family: Arial;
        font-size: 10pt;
    }
    .modalBackground
    {
        background-color: Black;
        filter: alpha(opacity=40);
        opacity: 0.4;
    }
    .modalPopup
    {
        background-color: #FFFFFF;
        width: 300px;
        border: 3px solid #0DA9D0;
    }
    .modalPopup .body
    {
        min-height: 20px;
        line-height: 20px;
        text-align: center;
        padding: 5px;
    }
    .modalPopup .footer
    {
        padding: 3px;
    }
    .modalPopup .button
    {
        height: 23px;
        color: White;
        line-height: 23px;
        text-align: center;
        font-weight: bold;
        cursor: pointer;
        background-color: #9F9F9F;
        border: 1px solid #5C5C5C;
    }
    .modalPopup td
    {
        text-align: left;
    }
</style>
<cc1:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
</cc1:ToolkitScriptManager>
<asp:LinkButton ID="lnkDummy" runat="server"></asp:LinkButton>
<cc1:ModalPopupExtender ID="ModalPopupExtender1" BehaviorID="mpe" runat="server"
    PopupControlID="pnlPopup" TargetControlID="lnkDummy" BackgroundCssClass="modalBackground"
    CancelControlID="btnNo">
</cc1:ModalPopupExtender>
<asp:Panel ID="pnlPopup" runat="server" CssClass="modalPopup" Style="display: none">
    <div class="body">Do you want to Save data?</div>
    <div class="footer" align="center">
        <asp:Button ID="btnYes" runat="server" Text="Yes" OnClick="OnYes" />
        <asp:Button ID="btnNo" runat="server" Text="No" />
    </div>
</asp:Panel>
Item QTY : <asp:Label ID="lblItemQty" runat="server" Text="6"></asp:Label><br />
Pack QTY : <asp:Label ID="lblPackQty" runat="server" Text="6"></asp:Label><br />
<asp:Button ID="btnSave" runat="server" OnClick="OnSave" Text="Save" />

Code

C#

public void OnSave(object sender, EventArgs e)
{
    // Check your condition.
    if (lblItemQty.Text == lblPackQty.Text)
    {
        ModalPopupExtender1.Show();
    }
}
public void OnYes(object sender, EventArgs e)
{
    //insertbalgv();
    ClientScript.RegisterClientScriptBlock(this.GetType(), "", "alert('Record Inserted!')", true);
}

VB.Net

Public Sub OnSave(ByVal sender As Object, ByVal e As EventArgs)
    ' Check your condition.
    If lblItemQty.Text = lblPackQty.Text Then
        ModalPopupExtender1.Show()
    End If
End Sub

Public Sub OnYes(ByVal sender As Object, ByVal e As EventArgs)
    'insertbalgv()
    ClientScript.RegisterClientScriptBlock(Me.GetType(), "", "alert('Record Inserted!')", True)
End Sub

Screenshot