private void BtnPurchase(object sender, RoutedEventArgs e)
 {
     if (!txtTotVatInc.Text.Equals(""))
     {
         if (!txtAmout.Text.Equals(""))
         {
             if (Double.Parse(txtTotVatInc.Text) < Double.Parse(txtAmout.Text))
             {
                 txtChange.Text = da.Buy(Double.Parse(txtTotVatInc.Text), Double.Parse(txtAmout.Text)).ToString();
                 txtChange.Text = (Math.Round(Double.Parse(txtChange.Text), 2)).ToString();
                 BuyFinished    = true;
                 da.DecreaseUnitInStock();
                 da.SaveInvoice(idlog);
             }
             else
             {
                 MessageBox.Show("Insuficient amount please add");
             }
         }
         else
         {
             MessageBox.Show("Please enter amount rendered");
         }
     }
     else
     {
         MessageBox.Show("Please scan items");
     }
 }