public class ViewModel: INotifyPropertyChanged
{
    public string WrappedProperty
    {
        get => domainObject.Property;
        set
        {
            domainObject.Property = HttpUtility.DecodeHtml(value);
            //Raise Property changed event etc
        }
    }
}

Binding binding = new Binding("HtmlText", _viewModel, "WrappedProperty", true, DataSourceUpdateMode.OnPropertyChanged,null,null);
_richEditControl.DataBindings.Add(binding);