Charlie Sheen's dream of turning his Beverly Hills mansion into a porn house could come true -- TMZ has learned a MAJOR porn company has made an official CASH offer on the place.



Sources connected to the deal tell us ... the bigwigs at YouPorn.com approached Sheen's realtor  with an offer to buy the place for $4.5 million in cash .... WAY lower than Sheen's listing price, $7.2 mil.

We're told the YouPorn people have told Sheen's people they hope to fulfill Sheen's dream of moving in several porn stars to live at the mansion. The YouPorn peeps told the realtor if Sheen accepts the deal, he can hang out at the porno compound as much as he wants.

So far, we're told Sheen has not made a decision on the offer.