President Carter's brother, Billy, walked into the offices of a Florida-based oil company several months ago and suggested that he might be able to help it obtain more crude oil from Libya.

A spokesman for the corporation, a fast-growing conglomerate called the Charter Co., gave that account yesterday in response to disclosures in Billy Carter's registration statement as a foreign agent for the Libyan government. cThe company didn't go to Carter, the spokesman said; Carter came to the company.

In the registration statement filed Monday in response to a court order, Carter said that he was not currently engaged in any activities on behalf of the Libyans. But he said that he was seeking to secure an increase in the amount of crude oil that a Charter subsidiary is permitted to buy from the National Oil Corp. of Libya.

Carter said that he and a colleague, Henry R. Coleman of Plains, Ga., had been involved in discussions with Libyan government officials about the matter "commencing in March 1979." He said no agreement had been reached, but that the Charter Co. had said "it will pay him a commission should it receive an increase in its oil allocation from Libya."

Charter Co. vice president and spokesman Park L. Beeler said, however, that Carter first contacted officials of his company in March of this year.

"He did call on our crude oil department and he did suggest that we might be able to obtain more crude" from Libya, Beeler said. "He asked us if we would be interested and we said 'Yes, we would.' He came personally to our offices in Jacksonville."

The Charter Oil Co., the Charter Co. subsidiary, has been obtaining crude oil from Libya since last year when it brought the Carey Energy Co., the biggest independent petroleum marketer on the East Coast.

Carey, which then owned 65 percent of a huge, 500,000-barrel-a-day refinery in the Bahamas, was almost bankrupt before the sale and reportedly owned $180 million to the Libyans for oil supplied to the refinery.

Carey has been getting about 100,000 barrels a day from Libya, but the Libyans had reportedly decided to cut off the supply since the company was not paying its bills. When Carter entered the picture, the Libyans agreed to continue selling the oil to the new company under a special pricing arrangement that included payment on the debts.

Charter Oil's contract with Libya ran out April 30, the month after Carter approached the company. The firm had been getting 100,000 to 125,000 barrels a day of Libya's low-sulfur crude, but the government limited it under the new contract to 60,000 barrels a day, Beeler said.

He said this was not unusual. "Libya has pretty much cut [its supplies to all oil companies] across the board," Beeler said.

He said Charter Oil executives asked Carter in March to supply more specifics about his proposal, but nothing has come of it so far. Beeler said "no agreements were signed and no agreements were made."

The Charter Co., which also has extensive holdings in insurance, publishing and other fields, had sales last year of $4.3 billion. Most of the revenues, however, come from oil. Its successful turnaround of Carey's operations sent its stock soaring, from about $5 a share early in 1979 to a high of $50.

Billy Carter told reporters yesterday in Americus, Ga., that he had registered as a foreign agent "under protest" and that he was now looking for ways to repay $220,000 that he had borrowed from the Libyan government.

"I spent all my time the past year in the drunk tank or the grand jury," he said, alluding to his medical treatment in California for alcoholism and to a federal investigation of the Carter family peanut business in Plains.

He said he did not feel he had done anything to qualify as an agent of the Libyan government, but filed the registration statement because he believed the Justice Department was under pressure to prosecute him. He said he feared that officials at Justice would have sought an indictment if he had not agreed to register.

The American Jewish Committee issued a sharp criticism of the Libyans' payments to Carter, which included almost $3,000 in gifts and almost $16,000 in travel expenses as well as the $220,000 he said he got this year as installments on a promised $500,000 loan.

"It is obvious that the Libyan government did not pay Billy Carter to act on its behalf because of his knowledge of public relations or his business acumen," the committee said in a statement. "It did so, because Billy is the brother of the president of the United States.

"President Carter should reject, publicly and unequivocally, this flagrant attempt to misuse the office of the president of the United States," declared AJC president Maynard I. Wishner.

Billy Carter told reporters in Americus that he had never discussed Libya, or the controversy over whether he should register as a foreign agent, with the president.

"He's my brother, but I don't tell him what to do and he doesn't tell me what to do," Carter said, echoing past statements concerning his activities.

Charter spokesman Beeler was vague about what kind of a commission Carter was to be paid or how much additional oil Carter thought the company might obtain. He maintained the approach was no different from those Charter gets every day from any oil broker.

Oil brokers, howover, do not normally arrange direct sales by national oil companies to refining companies, according to oil industry sources.

"If you deal with a broker, it's understood a fee is involved," Beeler said. But he said he did not know whether a specific amount or percentage was mentioned in the meeting with Carter. "I wasn't at the meeting," he said.

Charter Oil is currently refining between 150,000 and 180,000 barrels a day at the Bahamas refinery, which has a potential for 250,000 barrels. Standard Oil Co. of California owns the other half of the refinery. Charter has about a half-dozen crude oil suppliers for its operations there, but Libya is the biggest.