<DataGridTemplateColumn   Header="Валюта">
    <DataGridTemplateColumn.CellTemplate>
        <DataTemplate>
            <TextBlock Text="{Binding Path=ExchangeRates.Title}"/>
        </DataTemplate>
    </DataGridTemplateColumn.CellTemplate>
    <DataGridTemplateColumn.CellEditingTemplate>
        <DataTemplate>
            <ComboBox ItemsSource="{Binding Path=DataContext.ExchangeRates,
              RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}}}"
              SelectedItem="{Binding Path=ExchangeRates, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
               DisplayMemberPath="Title" IsSynchronizedWithCurrentItem="True"/>
        </DataTemplate>
    </DataGridTemplateColumn.CellEditingTemplate>
</DataGridTemplateColumn>

/*ExchangeRates коллекция для ComboBox
Тут была попытка получить элемент из коллекции и потом его поставить в качестве выбранного(не помогло)
ModelModul.ExchangeRates ex = ExchangeRates.Single(objEx =>
 objEx.Id == product.ExchangeRates.Id);
ExchangeRates.Contains(ex) хотя тут вернул true*/
PurchaseInfos.Add(new PurchaseInfos
{
    Products = new Products
    {
        Barcode = product.Barcode,
        Groups = product.Groups,
        //ExchangeRates = ex,
        ExchangeRates = product.ExchangeRates,
        Id = product.Id,
        IdExchangeRate = product.IdExchangeRate,
        IdGroup = product.IdGroup,
        IdUnitStorage = product.IdUnitStorage,
        IdWarrantyPeriod = product.IdWarrantyPeriod,
        PurchasePrice = product.PurchasePrice,
        SalesPrice = product.SalesPrice,
        VendorCode = product.VendorCode,
        UnitStorages = product.UnitStorages,
        WarrantyPeriods = product.WarrantyPeriods,
        Title = product.Title,
        SerialNumbers = new ObservableCollection<SerialNumbers>()
    },
    IdProduct = product.Id,
    IdExchangeRate = product.IdExchangeRate,
    ExchangeRates = product.ExchangeRates
});

public override bool Equals(Object obj)
{
    if (obj == null || !GetType().Equals(obj.GetType()))
    {
    return false;
    }
    return Id == ((ExchangeRates)obj).Id &&
             Title == ((ExchangeRates)obj).Title && Course == ((ExchangeRates)obj).Course;
}