There is no value which displayed in the Target text box on change envent

private void AmountPaid_TextChanged(object sender, System.EventArgs e)
{
     Regex regex = new Regex("^[0-9]*&");
    if(regex.IsMatch(this.AmountPaid.Text) && !string.IsNullOrEmpty(this.AmountPaid.Text.Trim()))
    {
    double Balance1;
    double Amount1 = Convert.ToDouble(this.Amount.Text);
    double Amount2 = Convert.ToDouble(this.AmountPaid.Text);

        Balance1 = Amount1 - Amount2;
        //Convert.ToDouble(this.Balance.Text)= Balance1;
        this.Balance.Text = Convert.ToString(Balance1);

    }
}

using Microsoft.VisualBasic;
using BaseClasses.Web.UI.WebControls;
using System;
using System.Collections;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using BaseClasses;
using BaseClasses.Data;
using BaseClasses.Utils;
using ReportTools.ReportCreator;
using ReportTools.Shared;
using System.Text.RegularExpressions;