Donald Trump wasn't laughing when funnyman Jerry Seinfeld canceled an upcoming appearance at a benefit for his son's Eric Trump Foundation. Seinfeld pulled out of the Sept. 13 event benefiting the St. Jude Children's Research Hospital because he disagrees with The Donald's questioning of Barack Obama's citizenship, his reps said. "I just learned you canceled a show for my son's charity," Trump fired off in a scorching letter to Seinfeld yesterday, "because of the fact that I am being very aggressive with respect to President Obama, who is doing an absolutely terrible job as our leader." "Celebrity Apprentice" star Trump piled on the insults, saying he wished he'd canceled his appearance on Seinfeld's "failed" NBC show, "The Marriage Ref." "We don't care that you broke your commitment," he wrote, "even though the children of St. Jude are very disappointed, and despite the fact that your manager clearly stated you are 'truly a man of his word.' "What I do feel badly about is that I agreed to do, and did, your failed show, 'The Marriage Ref,' even though I thought it was absolutely terrible . . . Despite its poor ratings, I didn't cancel on you like you canceled on my son and St. Jude. I only wish I did." The letter ends with the kiss-off, "You should be ashamed of yourself!" Back in January, before Trump announced a possible presidential run, Seinfeld agreed to perform at the foundation event at the Trump National Golf Club in Briarcliff Manor. But "Jerry has grown increasingly uncomfortable" with Trump's questioning Obama's citizenship, said a Seinfeld rep. "Jerry . . . feels this kind of demagoguery has no place in public discourse . . . He has respectfully withdrawn from the event, and is making a contribution both to the Eric Trump Foundation and to [St. Jude]." Now the Trumps won't need to fulfill Seinfeld's snack-heavy rider, which included demands for Hershey's Almond Kisses, original Fig Newtons and cold vegetable trays (no baby carrots). Inexplicably, Jerry also insisted on "NO CUSHIONED STOOLS!" Bret Michaels is taking over his slot.