The Federal Home Loan Bank Board is investigating Marvin L. Warner, a multimillionaire businessman, sports promoter and former ambassador, in connection with the purchase of stock in Freedom Savings and Loan of Tampa, the FHLBB announced yesterday. The transaction, which occurred during the past year, is reported to have benefitted Warner by several million dollars.

The investigation focuses on whether Warner and companies in which he held an interest failed to disclose in the proxy statement to shareholders of the S&L that Warner and the other companies were acquiring more than 10 percent of its stock or voting rights.

Should the board find he has wilfully violated the Change in Savings and Loan Control Act, Warner could be fined up to $10,000 a day or required to sell his stock.

The complicated deal also involved American Savings and Loan of Miami, ComBanks Corp. and Home State Financial Services, all companies that Warner either controls or has a financial interest in.

The board's case dates to Nov. 30, 1982, when Freedom's management recommended to stockholders that it buy six commercial banks in Orlando from ComBanks, a holding company whose chairman and controlling shareholder is Warner.

Management failed to get the two-thirds affirmative vote required because some major stockholders thought the price was about $10 million too high. After three unsuccessful tries, management got the required majority at a meeting on Jan. 4 to buy ComBanks' assets for $57.5 million.

What the shareholders did not know was that, on the previous day, Home State Financial Services--also controlled by Warner--had bought 200,000 shares, or 7.55 percent, of Freedom's stock, plus the right to direct its vote. Another 4.68 percent of Freedom's shares were voted at the direction of ComBanks by American Savings and Loan of Miami. Warner then owned 9.4 percent of American Savings' stock, and has increased his holdings to 28 percent.

The American Banker estimated that the recent sale of ComBanks, plus the sale of two other banking holding companies, benefited Warner by nearly $100 million.

Some of the major stockholders were quoted in published reports as opposing the transaction on the basis of cost and arguing that Warner had not disclosed his interests to them or filed a notice of change of control with the bank board.

Warner, 63, made his mark in business in Cincinnati real estate. He began acquiring interests in Florida in 1976. Warner has been active in Democratic politics and served as President Carter's ambassador to Switzerland from 1977 to 1979. A former part owner of the New York Yankees and Tampa Bay Buccaneers, Warner is now owner and chairman of the Birmingham, Ala., Stallions of the U.S. Football League.