This is actually a much simpler process than you may think., especially if you are on the Board. There is nothing that you need to do at this moment. Let him run through the course of selling his property, however, once a closing is scheduled and the attorneys are preparing their final paperwork, they will request a "paid assessment letter." This is where you begin to fight back and, by law, refuse to issue this letter, as assessments are, in fact, not paid. Needless to say, this will delay his closing but that is no matter to you, as recouping your funds is your priority.
As someone else answered, it will not hurt to give a heads up to both attorneys that this is the situation. The Board should have been furnished a copy of the contract by now, so all of this information should be in hand for your use.
I am the president of the my condo Board and have handled this type of situation, unfortunately, many times over. But, you and your association are protected.
FYI, filing a lien and going through the motions of taking possession of his unit in leiu of past-due assessments takes many, many months and your greates relief should lie in the sale of his unit. Honestly, this is the best way it could work out for your entire association, as it is most often the best way to get your accounts settled.
Hope this puts you at ease. In agreement with most, I do recommend that you consult a real estate attorney on behalf of your association.