Allegheny Beverage Corp. said yesterday that the buyer of its Service America subsidiary has reduced its offer to $450 million from $500 million.

The price was lowered after a weekend of telephone calls between Allegheny's Cheverly headquarters and the New York offices of Morgan, Lewis, Githens & Ahn, the investment banking firm that is working with senior management of Service America to buy the food service subsidiary.

The board of directors of Allegheny is scheduled to meet today to consider the reduced price. Allegheny shareholders, who last week gave preliminary approval to the Service America sale, must also approve the new price, according to Morton M. Lapides, Allegheny's chairman.

Officials of Servam, the purchasing group, said last week that profit projections for Service America had been reduced. That raised questions about Servam's ability to finance the purchase at $500 million.

The change in price would mean that the company, which had originally expected to receive the equivalent of about $21 a share based on the $500 million price, will now get about $14 or $15 a share, according to analysts. The stock closed yesterday at $11.37 1/2 , up 62 1/2 cents from the previous close, with 280,000 shares traded. Last week, when the annual meeting of the company was held, the company's stock was trading around $14.50.

At a reconvened annual meeting of shareholders yesterday, Lapides reiterated that there are no guarantees that the sale of Service America will be completed.

The company has repeatedly said that if the sale of Service America is not completed by Oct. 31, it will be unable to make a $15 million payment due its lenders. In addition, other loans may come due if the company defaults.

Because of Allegheny's need to pay off its loans, its lenders and Servam are now in control of the deal, said Richard Jones, a stock analyst with the Washington firm of Johnston, Lemon & Co. "But it's still an attractive offer from the shareholders' standpoint," he said.

In addition, the $450 million sale price would still allow the company to pay off its debts and have a considerable amount of cash to put into new businesses, Jones said. Company officials have said they have not determined what other businesses they might buy if the sale is completed. Service America is the last operating subsidiary of Allegheny, a once-larger company that has sold off its other subsidiaries.

The sale remains contingent on completion of financing by Servam and the company's ability to obtain consent of the holders of the majority of its outstanding 9 1/2 percent bonds.

Allegheny yesterday also announced that it would extend its offer of $1,050 for its $1,000 bonds until Oct. 23 at 6 p.m. The offer was to expire last night.