[WPF] Как в DataGrid очистить ячейку с форматом Decimal в окне пользователем?

 public void LoadMatlTable()
        {
            string sql = 
                "SELECT item AS item"
                + " ,[description] as [description]"
                + " ,[matl_qty] as MatlQty"
                + " ,u_m AS UM"
                + " ,[qty_expence] AS QtyExpence"
                + " ,flag AS flag"
                + " ,qty_issued AS QtyIssued"
                + " ,job AS job"
                + " ,suffix AS suffix"
                + " ,oper_num AS OperNum"
                + " ,whse AS whse"
                + " ,UserID AS UserID"
                + " ,Id AS Id"
                + " FROM MyTable" ;
            SqlConnection connection = null;
            try
            {
                //DataTable MatlTable = new DataTable();
                //SqlDataAdapter MatlAdapter = new SqlDataAdapter();
                MatlTable.Clear();
                connection = new SqlConnection(connectionString);
                var sqlCmd = new SqlCommand(sql, connection);
                MatlAdapter.InsertCommand = new SqlCommand("NMwpf_InsertPendingMaterialSp", connection);
                MatlAdapter.InsertCommand.CommandType = CommandType.StoredProcedure;
                MatlAdapter.InsertCommand.Parameters.Add(new SqlParameter("@job", SqlDbType.NVarChar, 50, "job"));
                MatlAdapter.InsertCommand.Parameters.Add(new SqlParameter("@suffix", SqlDbType.Int, 50, "suffix"));
                MatlAdapter.InsertCommand.Parameters.Add(new SqlParameter("@oper_num", SqlDbType.Int, 0, "OperNum"));
                MatlAdapter.InsertCommand.Parameters.Add(new SqlParameter("@whse", SqlDbType.NVarChar, 0, "whse"));
                MatlAdapter.InsertCommand.Parameters.Add(new SqlParameter("@UserID", SqlDbType.Int, 0, "UserID"));
                SqlParameter parameter = MatlAdapter.InsertCommand.Parameters.Add("@Id", SqlDbType.Int, 0, "Id");
                parameter.Direction = ParameterDirection.Output;
                connection.Open();
                MatlAdapter = new SqlDataAdapter(sqlCmd);
                MatlAdapter.Fill(MatlTable);
                MatlTable.Columns["QtyExpence"].AllowDBNull = true;
                MaterialGrid.ItemsSource = MatlTable.DefaultView;
                MatlTable.ColumnChanged += new DataColumnChangeEventHandler(Qty_Change);
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
            finally
            {
                if (connection != null)
                    connection.Close();
            }
        }