Normal maps are finicky things: Often it seems that the green channel needs to be flipped to be viewed properly in Maya relative to how it should be in your game engine. I've written tools to do this, but a modeler just clued me in: Maya has this built in, it's just a bit buried:
Select the shape node of your mesh, and in the Attribute Editor under the 'Tangent Space' frame, you can swap the 'Coordinate System' from 'Right Handed' to 'Left Handed', which effectively reverses the green channel of the normal map.
Select the shape node of your mesh, and in the Attribute Editor under the 'Tangent Space' frame, you can swap the 'Coordinate System' from 'Right Handed' to 'Left Handed', which effectively reverses the green channel of the normal map.