Sean Penn’s performance as a mentally challenged father battling for custody of his daughter in ”I Am Sam” is already getting Oscar buzz, but the movie, cowritten and directed by Jessie Nelson (”Corrina, Corrina”) looked like anything but a slam dunk when it was being developed at Fox several years ago. According to one of the filmmakers, Fox was unwilling to make the $25 million movie with Penn, who had written a scathing letter to the company after being denied the use of a corporate jet to attend a screening for 1998’s ”The Thin Red Line.” A Fox spokesperson denies that Penn was the problem, citing ”general creative differences [that made us] unable to move forward on the movie.” Last year, Sam jumped to New Line, with Michelle Pfeiffer attached to costar as a hard-charging lawyer. But then Michael DeLuca, who had greenlit the film, left the company, and a few weeks later, the day before filming began, New Line announced layoffs that included the executive on the film, Claire Polstein (who stayed on as a producer). On the bright side, says the filmmaker, ”We didn’t get a single note in postproduction. We stayed just under the radar.” Sam will receive an Oscar-qualifying limited run Dec. 28, opening nationally in January.