สอบถามปัญหาครับ อยากให้เวลาคืน ทำการคืนได้ครั้งเดียวครับ แก้ไขยังไงครับ มีตัวอย่างโค้ดครับ

Imports System.Data.SqlClient
Public Class frm_return
    Private Sub frm_return_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        txt_return.Text = frm_main.lbl_admin_name.Text
        refresh_data_return()
    End Sub
    Private Sub get_auto_num_char()
        sql = "select max(return_id) from tb_return"
        Try
            Dim numchar As String = "RET-" & (cmd_excuteScalar() + 1).ToString.PadLeft(6, "0")
            txt_return_id.Text = numchar
        Catch ex As Exception
            txt_return_id.Text = "RET-000001"
        End Try
    End Sub
    Private Sub Timer1_Tick(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Timer1.Tick
        lbl_borrow_day.Text = DateTime.Today
    End Sub

    Private Sub txt_return_borrow_id_KeyDown(ByVal sender As System.Object, ByVal e As System.Windows.Forms.KeyEventArgs) Handles txt_return_borrow_id.KeyDown
        If e.KeyData = Keys.Enter Then
            sql = "select borrow_device_id,borrow_device_name,borrow_user_id,borrow_user_name,borrow_admin,borrow_date_checkout,borrow_date_checkin from tb_borrow where borrow_full_id_2 = '" & txt_return_borrow_id.Text & "'"
            Dim dts As DataTable = cmd_excuteToDataTable()
            If dts.Rows.Count <= 0 Then
                error_message("ไม่พบรหัสอุปกรณ์")
            Else
                txt_return_device_id.Text = dts.Rows(0)("borrow_device_id")
                txt_return_device_name.Text = dts.Rows(0)("borrow_device_name")
                txt_return_user_id.Text = dts.Rows(0)("borrow_user_id")
                txt_return_user_name.Text = dts.Rows(0)("borrow_user_name")
                txt_return_admin_name.Text = dts.Rows(0)("borrow_admin")
                txt_dayoff.Text = dts.Rows(0)("borrow_date_checkin")
                txt_return.Select()
            End If
        End If
    End Sub
    Private Sub refresh_data_return()
        sql = "select return_full_id,return_borrow_full_id,return_device_id,return_device_name,return_user_id,return_user_name,return_admin_recieve,return_dayoff,return_day,return_status from tb_return"
        dtg_return.DataSource = cmd_excuteToDataTable()
        get_auto_num_char()
        chang_all_column()
    End Sub
    Private Sub chang_all_column()
        Dim header() As String = {"รหัสการคืน", "รหัสการยืม", "รหัสอุปกรณ์", "ชื่ออุปกรณ์", "รหัสพนักงาน", "ชื่อพนักงาน", "ผู้รับคืนอุปกรณ์", "กำหนดวันคืน", "วันที่คืน", "สถานะ"}
        chang_name_datagrid(dtg_return, header)
    End Sub
    Private Sub update_data_device()
        sql = "update tb_device set device_amount = device_amount + 1  where device_id = '" & txt_return_device_id.Text & "'"
        If cmd_excuteNonquery() = 0 Then
            error_message("ทำรายการไม่สำเร็จ")
        Else
            ok_message("ทำรายการสำเร็จ")
        End If
    End Sub
    Private Sub bt_return_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles bt_return.Click
        If confirm("คุณต้องการคืนอุปกรณ์") = vbNo Then Return
        sql = String.Format("insert into tb_return values ('{0}','{1}','{2}','{3}','{4}','{5}','{6}','{7}','{8}','{9}','{10}')",
                           txt_return_id.Text,
                           txt_return_borrow_id.Text,
                           txt_return_device_id.Text,
                           txt_return_device_name.Text,
                           txt_return_user_id.Text,
                           txt_return_user_name.Text,
                           txt_return.Text,
                           txt_dayoff.Text,
                           lbl_borrow_day.Text,
                           "คืน", "1")
        If cmd_excuteNonquery() = 0 Then
            error_message("ทำรายการไม่สำเร็จ")
        Else
            update_data_device()
            refresh_data_return()
        End If
    End Sub
End Class