You will have to pay half for it. My guess is that because your block hadn't settled yet, he didn't ask because he didn't know you owned the land. It's a bit cheeky, but you have to think of the upside...at least you don't have to organise that bit of fence.

I had a lot of dispute with a neighbour over a fence, and in the end it's not worth trying to fight it....they will be your neighbour! You don't have to pay for it till you move into your house (I don't think) and he has to provide you with the invoice to show how much that portion of fence cost, and your 50% share. If you can't afford it, then you can pay him off in installments.