/// <summary>
/// Update the source calling BindingExpression.Update for the specified property
/// </summary>
[Description("Update the source calling BindingExpression.Update for the specified property")]
public class UpdateSourceBindingAction : TriggerAction<FrameworkElement>
{
    public readonly static DependencyProperty PropertyProperty = DependencyProperty.Register("Property", typeof(String), typeof(UpdateSourceBindingAction), new PropertyMetadata(null, OnPropertyChanged));

    private DependencyProperty property;

    /// <summary>
    /// Initializes a new instance of the <see cref="UpdateSourceBindingAction"/> class.
    /// </summary>
    public UpdateSourceBindingAction()
    {
    }

    /// <summary>
    /// Gets or sets the property to update.
    /// </summary>
    /// <value>The property.</value>
    public String Property
    {
        get { return this.GetValue(PropertyProperty) as String; }
        set { this.SetValue(PropertyProperty, value); }
    }

    /// <summary>
    /// Called after the action is attached to an AssociatedObject.
    /// </summary>
    protected override void OnAttached()
    {
        base.OnAttached();

        SearchProperty();
    }

    private static void OnPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        UpdateSourceBindingAction u = (UpdateSourceBindingAction)d;
        u.SearchProperty();
    }

    private void SearchProperty()
    {
        if (this.AssociatedObject == null)
        {
            this.property = null;
            return;
        }

        string propertyName = this.Property + "Property";
        FieldInfo field = this.AssociatedObject.GetType().GetField(propertyName, System.Reflection.BindingFlags.FlattenHierarchy | System.Reflection.BindingFlags.Public | System.Reflection.BindingFlags.Static);
        if (field != null)
            this.property = field.GetValue(null) as DependencyProperty;
        else
            this.property = null;
    }

    /// <summary>
    /// Invokes the specified o.
    /// </summary>
    /// <param name="o">The o.</param>
    protected override void Invoke(object o)
    {
        if (property == null) return;

        BindingExpression be = this.AssociatedObject.GetBindingExpression(this.property);
        if (be != null)
            be.UpdateSource();
    }
}

<TextBox Text="{Binding Testo, Mode=TwoWay}">
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="TextChanged">
            <behaviors:UpdateSourceBindingAction Property="Text" />
        </i:EventTrigger>
    </i:Interaction.Triggers>
</TextBox>