Contracts on behalf of an incorporated association may be made as follows:

(a) a contract that, if made between natural persons, would be by law required to be in writing under seal may be made on behalf of the association in writing under the common seal of the association;

(b) a contract that, if made between natural persons, would be by law required to be in writing, signed by the parties to be charged therewith may be made on behalf of the association in writing signed by a person acting under its authority, express or implied;

(c) a contract that, if made between natural persons, would by law be valid although made by parol only (and not reduced into writing) may be made by parol on behalf of the association by a person acting under its authority, express or implied –

and a contract so made is effectual in law and binds the association and its successors and all other parties thereto, and may be varied or discharged in the manner in which it is authorized to be made.