I dont know about him getting his money back, does he have a document making his donation conditional for the sole use of building a new hall.
Also the hall would have done Motion and vote before sending the funds, what was his participation in that. Did you oppose the vote. If not they could say he now gave permission for the funds to be used for another purpose.
Plus like many said if he makes the suit he will face repercussions in the organization for it is he ready for that?