In his cameo, the star does whatever it takes to get donations for his real-life charity.
"The joke is that Matt is very passionate about his charity -- almost in a stalkerish kind of way," Grenier, who plays movie star Vince, said. "Vince becomes intimated by his intensity. He ends up bullying Vince into giving much more than he should."
Costar Emmanuelle Chriqui, who plays Sloan, added: "Matt gets Vince to make a massive donation. He's like, 'Bro whip out your checkbook!'"
How did Entourage persuade Damon to sign on for the part?
"It wasnt hard to get him," executive producer Doug Ellin said. "Matt came on the phone with me and wanted to do something that focused on his charity, One By One, which helps kids all over the world."
NBA player Lebron James also appears in the finale alongside Damon.
"Lebron and Matt have a scene together, and they just kill it!" Ellin said.