Hi makumbi,

Please refer below sample.

HTML

<table>
<tr>
    <td>Cash</td>
    <td>
        <asp:TextBox ID="txtCash" runat="server" OnTextChanged="Cash_TextChanged"></asp:TextBox>
    </td>
</tr>
</table>
<asp:Button ID="btnConfirm" runat="server" OnClick="OnConfirm" Style="display: none;" />
<script type="text/javascript">
    function Confirm(message) {
        var confirm_value = document.createElement("INPUT");
        confirm_value.type = "hidden";
        confirm_value.name = "confirm_value";
        if (confirm(message)) {
            confirm_value.value = "Yes";
        }
        else {
            confirm_value.value = "No";
        }
        document.forms[0].appendChild(confirm_value);
        document.getElementById('btnConfirm').click();
    }
</script>

Code

C#

public void Cash_TextChanged(object sender, EventArgs e)
{
    string value = txtCash.Text;
    string message1 = "You have enetered Less money by: " + value + "\\nIf this is a credit sale enter the account No of the Customer, Is it a credit Sale ?";
    ClientScript.RegisterStartupScript(this.GetType(), "alert", "Confirm('" + message1 + "');", true);
}

public void OnConfirm(object sender, EventArgs e)
{
    string confirmValue = Request.Form["confirm_value"];
    if (confirmValue == "Yes")
    {
        ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('You clicked YES!')", true);
    }
    else
    {
        ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('You clicked NO!')", true);
    }
}

VB.Net

Public Sub Cash_TextChanged(ByVal sender As Object, ByVal e As EventArgs)
    Dim value As String = txtCash.Text
    Dim message1 As String = "You have enetered Less money by: " & value & "\nIf this is a credit sale enter the account No of the Customer, Is it a credit Sale ?"
    ClientScript.RegisterStartupScript(Me.GetType(), "alert", "Confirm('" & message1 & "');", True)
End Sub

Public Sub OnConfirm(ByVal sender As Object, ByVal e As EventArgs)
    Dim confirmValue As String = Request.Form("confirm_value")

    If confirmValue = "Yes" Then
        ClientScript.RegisterStartupScript(Me.GetType(), "alert", "alert('You clicked YES!')", True)
    Else
        ClientScript.RegisterStartupScript(Me.GetType(), "alert", "alert('You clicked NO!')", True)
    End If
End Sub

Screenshot