Engine/NMPR

The problem you have here is a rsleut of the way that shadow pass is created.You are using the floor that don't cover the whole image.Take a look at the shadow pass alone:You'll notice that wherever you have alpha of 0 the shadow pass is black.In may opinion (I'm not the only one) this is a bug. In those areas shadow pass should be white.If it were as it should be, i.e. white your image wouldn't change in those areas (multiplying by white doesn't change image as you remember).Here we have antialiased edges of your object and you get dark fringe around it.The way to solve it is to take the alpha, pass it through Invert node, add it to shadow pass and use the rsleut as the shadow pass.