It seems that AIMCo wasn't the only one approached to make a rival bid. CBC reports that PotashCorp takeover bid to get review:
The head of a major Canadian pension fund says a Chinese sovereign wealth fund is interested in making a joint-bid with a Canadian pension fund for fertilizer-maker Potash Corp. of Saskatchewan Inc.
Leo de Bever, chief executive of Alberta Investment Management Corp., which manages $70 billion Canadian (US$65 billion) in public sector pension funds, said Thursday his fund and other major pension funds in Canada have been approached by an intermediary looking to put together a rival bid to BHP Billiton's $39 billion hostile offer.
De Bever said in an interview on Thursday that he's not interested.
Australia's BHP Billiton Ltd., the world's biggest mining company, launched a hostile $130-a-share takeover on Aug. 18 after Potash directors rejected its offer.