[Moo] morph.start(with variable) do not work

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Hello I have a problem to start my morphing with a variable.
I do the following:

            ff = "margin-left"

            i.tween = new Fx.Morph(i, {
                duration: 600
            }).start({
                ff: [0],         //   'margin-right' works
                'opacity': [1]
            });


if I use 'margin-right' instead of ff it works fine.

http://jsfiddle.net/H84d8/1/

whats wrong?
thx for any hint in advance
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