coercion

Improper use (or threat of improper use) of authority, economic power, physical force, or other such advantage, by a party to compel another tosubmit to the wishes of its wielder. Agreements entered into, or testaments signed, under coercion are considered illegal and invalid. See also duressand undue influence. 

The contract was nullified after it was found out that the union president had used coercion to made the company president sign it.