Greenmail

n. threat to control a company by buying a large amount of its shares (in order to sell them back to the company at an inflated price)

Greenmail or greenmailing is a corporate acquisition strategy for generating large amounts of money from the attempted hostile takeovers of large, often undervalued or inefficient companies.
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Noun
1. (corporation) the practice of purchasing enough shares in a firm to threaten a takeover and thereby forcing the owners to buy those shares back at a premium in order to stay in business
(hypernym) shark repellent, porcupine provision
(classification) corporation, corp

Situation in which a large block of stock is held by an unfriendly company, forcing the target company to repurchase the stock at a substantial premium to prevent a takeover.