First of all it's not an early marriage; she is 14 and as someone correctly stated above 14 is legal in some states in the US. And as I mentioned in my previous post it's widely practiced across the world and has been practiced in the West as well.
Secondly, the UN has a is heavily Westernised, that is, it perpetuates ideals held by secular nations where liberalism is the agenda. It doesn't recognised Sharia Law or indeed Islamic states like Nigeria. Bringing the U.N laws in to the discussion is hardly relevant, so spare me the theatrics.
Thirdly, you keep mentioning the words slave and slavery yet nowhere in the original article have I read that this 14 year old girl has been sold or indeed bought. Weird because you'd think if this was indeed the case the newspaper would've commented on it.