Its actually a PR (public relations) issue and I doubt anyone would give you and advice on this matter. Best would be you confront your neighbour and ask him in a calm and controlled manner as to when he is going to pay for the model. Let him know your side of the coin. It doesnt hurt to try. You can also speak to your landlord and appraise him of the issue beforehand so he doesnt have any misconceptions.

In our culture, we advocate never to bring money into friendships for this reason

Ameyam