In exchange, we ask to be able to announce our probono collaboration (in this website, in our twitter account, or in other social network) and that you do the same. Some examples would be a mention or link in your site, a blog post announcing your migration to WP and mentioning that we helped you in the process and/or publicity through your social sites (Twitter, Facebook, etc). You don´t need to do all these things (though that would be absolutely great!) but at least some of them (and at least one mention before starting the migration process).
*We reserve the right to freely decide which NGO/non-profit can benefit from our probono work. Third parties can be involved in the discussion (and if so we think it´s only fair they also do it probono!) but it is mandatory that the agreement to these conditions comes from a legal representative of the organization itself. Proof of the organization legal status could be requested.