As they say, people can sue for just about anything.  The question is whether they will win.

You seem to have put the proverbial rabbit in the hat by describing the suit as one to collect for a “contribution” of services. Unless you have a record of paying those who serve on the board, your disgruntled director is not likely to win.  If this person served for nine years without being paid, along side others who were not paid, without asking for payment, and without asking that an obligation be accrued on the organization’s financial statement, success does not seem likely.  About 90% of public charities do not pay for board service, and it will take some pretty strong evidence to remove you from that category.  This sounds more like a spiteful threat than a serious claim.