gistfile1.cs

public interface IFoo { void MyMethod(Action<int[]> callback); }

[Test]
public void TestCallback()
{
    var sub = Substitute.For<IFoo>();
    var callback = Substitute.For<Action<int[]>>();
    var data = new[] { 1, 2, 3 };

    sub.When(x => x.MyMethod(callback))
       .Do(x => callback(data));

    sub.MyMethod(callback);
    callback.Received()(data);
}