The Situation could have more market power than anyone imagined -- because a day after Abercrombie & Fitch offered him and the other "Jersey Shore" cast members cash NOT to wear its clothes ... the company's stock plummeted 9%.

Abercrombie's stock hit the new low today -- the first trading session since the company offered Sitch and the cast substantial $$$ to stop wearing A&F-branded products.

Abercrombie fired off a press release yesterday, offering the cast "a substantial payment" in return for not wearing its threads, claiming The Situation goes against the "aspirational nature of its brand."

A source directly connected with The Situation tells TMZ ... Abercrombie never personally contacted them, or in any way made a financial offer to stay away from A&F clothing.