public  class FieldTwoProperty : OnPropertyChangedClass
    {
        private string _name;

        public string Name
        {
            get { return _name; }
            set { _name = value; OnPropertyChanged(); }
        }

        private string _value;

        public string Value
        {
            get { return _value; }
            set { _value = value; OnPropertyChanged(); }
        }

        public override bool Equals(object obj)
        {
            var property = obj as FieldTwoProperty;
            return property != null &&
                   _name == property._name &&
                   _value == property._value;
        }

        public override int GetHashCode()
        {
            var hashCode = 179903332;
            hashCode = hashCode * -1521134295 + EqualityComparer<string>.Default.GetHashCode(_name);
            hashCode = hashCode * -1521134295 + EqualityComparer<string>.Default.GetHashCode(_value);
            return hashCode;
        }
    }

    public class Model
    {
        List<FieldTwoProperty> _listField;
        public Model()
        {
            _listField = new List<FieldTwoProperty>()
            {
                new FieldTwoProperty(){Name="Телефон",Value= "+0 (123) 456-78-90"},
                new FieldTwoProperty(){Name="Адрес",Value= "Новые Васюки"}
            };
        }
        public IEnumerable<FieldTwoProperty> GetFields() => _listField.ConvertAll(field => new FieldTwoProperty() {Name=field.Name, Value=field.Value });
        public void SetFields(IEnumerable<FieldTwoProperty> value) => _listField = value?.ToList();
        public bool IsEqualsField(IEnumerable<FieldTwoProperty> value)
        {
            if (_listField == null && value == null) return true;
            if (_listField == null || value == null || _listField.Count != value.Count()) return false;
            return value
                .Select((field, ind) => new { Field = field, Index = ind })
                .FirstOrDefault(fieldInd => !fieldInd.Field.Equals(_listField[fieldInd.Index]))
                == null;
        }
    }

 public   class ViewModel : OnPropertyChangedClass
    {
        private List<FieldTwoProperty> _listFields;

        public List<FieldTwoProperty> ListFields
        {
            get { return _listFields; }
            set { _listFields = value; OnPropertyChanged(); }
        }

        private Model model;

        public ViewModel()
        {
            model = new Model();
            ListFields = model.GetFields().ToList();
        }

        private ICommand _saveComm;
        public ICommand SaveComm => _saveComm ?? (_saveComm = new RelayCommand(OnSave));
        private void OnSave(object value)
        {
            model.SetFields(value as List<FieldTwoProperty>);
            ListFields = model.GetFields().ToList();
        }

        public bool IsNotModified => model.IsEqualsField(_listFields);
        public bool IsModified => !IsNotModified;

        public void Changed() => OnPropertyChanged("IsNotModified IsModified");
    }

<Window x:Class="ChangedSourceInDatagrid.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:ChangedSourceInDatagrid"
        mc:Ignorable="d" SizeToContent="Height"
        Title="MainWindow" Width="300">
    <d:Window.DataContext>
        <local:ViewModel>
            <local:ViewModel.ListFields>
                <local:FieldTwoProperty Name="Поле 1" Value="Значение 1"/>
                <local:FieldTwoProperty Name="Поле 2" Value="Значение 2"/>
            </local:ViewModel.ListFields>
        </local:ViewModel>
    </d:Window.DataContext>
    <local:ContextHolder>
        <local:ContextHolder.Commands>
            <local:CommandBinding RoutedCommand="{x:Static ApplicationCommands.Save}"
                                  RelayCommand="{Binding SaveComm}"/>
        </local:ContextHolder.Commands>
        <StackPanel>
            <DataGrid x:Name="DG"  Margin="5" ItemsSource="{Binding ListFields, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" ColumnWidth="*" 
                      SelectedCellsChanged="DG_SelectedCellsChanged"
                      LostFocus="DG_LostFocus"/>
            <Button Margin="5" HorizontalAlignment="Center" IsEnabled="{Binding IsModified}" 
                    Command="{x:Static ApplicationCommands.Save}"
                    CommandParameter="{Binding ListFields}">Сохранить</Button>
        </StackPanel>
    </local:ContextHolder>
</Window>

    public partial class MainWindow : Window
    {
        private ViewModel viewModel=new ViewModel();
        public MainWindow()
        {
            InitializeComponent();
            DataContext = viewModel;
        }

        private void DG_SelectedCellsChanged(object sender, SelectedCellsChangedEventArgs e)
        {
            viewModel.Changed();
        }
        private void DG_LostFocus(object sender, RoutedEventArgs e)
        {
            viewModel.Changed();
        }
    }