Rhino Mocks receive argument, modify it and return?

You could use the WhenCalled method like this:

myStub
    .Stub(_ => _.Create(Arg<Invoice>.Is.Anything))
    .Return(null) // will be ignored but still the API requires it
    .WhenCalled(_ => 
    {
        var invoice = (Invoice)_.Arguments[0];
        invoice.Id = 100;
        _.ReturnValue = invoice;
    });

and then you can create your stub as such:

Invoice invoice = new Invoice { Id = 5 };
Invoice result = myStub.Create(invoice);
// at this stage result = invoice and invoice.Id = 100