Over the objections of European investors, the board of trustees of Ameribanc Investors Group has voted to issue $28.75 million worth of preferred stock to an investment partnership headed by two prominent Washington investors, former U.S. Ambassador J. William Middendorf II and Cyrus A. Ansary.

Ameribanc Investors Group is the parent of Ameribanc Savings Bank of Annandale, a savings and loan with assets of $857 million and 30 offices in Virginia.

The Middendorf-Ansary purchase, in effect, will give their partnership a 29 percent stake in Ameribanc and will reduce from about 35 percent to 24 percent the shares represented by trustee Peter S. Zwart, a European investment adviser.

Zwart has said he opposed the preferred stock agreement, believing that the sale of the bank would be the best way to maximize the value of Ameribanc shares. "We continue to think that it would represent the stockholders' interest better to pursue a business combination," he said.

Zwart declined to comment when asked whether he would sue to prevent the stock sale.

Zwart sought unsuccessfully last year to encourage NCNB Corp. of North Carolina, a bank holding company, to acquire Ameribanc.

After Ameribanc's management and board declined to pursue the merger, Zwart told the Ameribanc stockholders' meeting in May he would continue his fight to sell the bank.

Because Zwart's efforts represented a potential threat to current management, the preferred stock sale was a victory for William H. Savage, president and chief executive officer of Ameribanc Investors.

Said Savage, "Mr. Zwart is entitled to his opinion. Reasonable men can differ."

Although the preferred stock is not convertible to common shares for two years, it could be voted in the event of a vote on control of the company. The convertible shares will carry a dividend of 5.5 percent a year.

The Ameribanc trustees also approved an aggressive business plan for the S&L. "Both by internal growth and by acquisitions, Ameribanc intends to expand beyond its Washington metropolitan area and Virginia bases into Maryland and the Carolinas," Savage said.

Savage said he expected that the bill providing for the recapitalization of the Federal Savings and Loan Insurance Corp., now awaiting President Reagan's signature, would create opportunities for Ameribanc to acquire troubled S&Ls in Virginia and in suburban Maryland.

Ansary said that he and Middendorf would be joined in their investment partnership by Bertram Cohn, a partner in First Manhattan Co., a New York investment house, and by George D. O'Neill, a director of the Rockefeller Group, also in New York.

Ansary said he foresaw "significant expansion" of the S&L's operations in the next five years.

Savage said that the financing plan was worked out with the help of Johnston, Lemon & Co., a regional investment firm. As a result, Ameribanc trustee Stephen Hartwell, a Johnston, Lemon officer, abstained from voting.

Middendorf was most recently U.S. ambassador to the European Community. Ansary is a Washington attorney, chairman of the board of American University and a director of First American Bank of Maryland.