Heavyweight challenger Trevor Berbick today said he would not fight Muhammad Ali as scheduled Friday night unless the promoter, James Cornelius, guaranteed he would be paid.

"I need a guarantee before I get in the ring," Berbick said at today's weigh-in. "I did my part, and I'm sure Brother James will do his part."

Cornelius, accused by rival promoter Don King of assaulting him Sunday night, was clearly agitated as Berbick made his demands. Finally, as questions about money kept coming, Cornelius shouted at a reporter: "Stay out of our business. If you want to ask questions, ask questions about the damned fight."

This now is the trauma, not the drama, in Bahama.

At 39, Ali weighed in at 236 1/2 pounds, the heaviest of his 20-year career, nearly 20 pounds of the jiggly stuff that wasn't there in his humiliation of 14 months ago against Larry Holmes. So sorry did Ali look then, with only minimal improvement in six weeks training here, that Berbick was asked if he would let up on the old champion if he saw him in trouble.

"Sure," said Berbick, who lost with honor to Holmes in April, "you're not here to try to kill somebody."

At 28, weighing 216, Berbick would take a 19-2-1 record against Ali's 57-4. Berbick is an aggressive strongman who will force the action against an aging Ali who still talks lickety-split -- "I will rock the world" -- but whose only chance of victory seems to be keeping Berbick away with his jab for the full 10 rounds.

Ali first won the heavyweight championship in 1964, won it again 10 years later, regained it in 1978 and believes a victory over Berbick would give him another shot at the title.

"I'm going to make my critics eat their words," he said today, explaining why he was so quiet. "This is too serious to play . . . I picked a tough guy. He gave Larry Holmes a butt-whipping. If I can beat this guy, I'm back."

Other fights here Friday: Thomas Hearns will work again after losing to Ray Leonard, meeting journeyman Ernie Singletary; high-ranking heavyweight Greg Page will take perhaps the last step to the title if he defeats veteran Scott LeDoux, and Earnie Shavers, denied a license to fight in the U.S. because of an eye problem, will meet young Jeff Sims.

All fighters are to be paid, supposedly, from cable television money. But Cornelius' refusals to talk about financing raised doubts the money is there. Berbick's public demands today, reiterated in an interview later, raised a possibility the promotion could sink in a sea of red ink.

Ticket sales are nil. The "stadium" seating has been cut from 18,000 to 11,000 on folding chairs and wooden bleachers on a baseball field. Bahamians can buy tickets at $10 instead of the advertised minimum of $50.

Meanwhile, a legal source involved in the promotion says several fighters have not been paid and others have taken paycuts.

Ali claims to have been paid $4 million, but Page's attorney, J. Bruce Miller, insists Ali's paycheck is a flat $1 million. Estimates of Berbick's pay top out at $250,000, with $100,000 for training expenses. Berbick says he has received only expense money.

Cornelius, who says he is a first-time fight promoter from Los Angeles with unidentified business connections here, stood next to Berbick as the fighter told reporters he woud feel better mentally about the fight "when they get my money up."

"It's no problem," Cornelius said about Berbick's money before cutting off questions.

Later, walking to his hotel, Berbick said he anticipated no trouble with Cornelius but added, "They have a little problem. They have to come up with all the money, even if they have to go break the bank. . . they've already violated four or five parts of the contract dealing with training facilities, food and transportation."

The undefeated Page has been paid already, according to attorney Miller. "Either Cornelius came up with the letter of credit to Chase Manhattan, or we walked yesterday," Miller said today. "We checked with Chase and it's there.

It will take some such guarantee to please Berbick, who laughed out loud when a reporter asked if Berbick would accept Cornelius' word as his guarantee.

"Are you crazy?" Berbick said. "In the bank."

Berbick, who handles his own negotiations, said he set no deadline for Cornelius to deliver the guarantee other than to say it necessarily would have to be before the banks closed Friday.

"There's a lot of money around here," Berbick said. "They can go rob a bank."