The Alexandria City Council warned officials with Virginia's state pension fund last night that a plan to have the fund become majority owner of valuable land south of National Airport might not be as lucrative a proposition as it seems.

On a 5 to 2 vote, the council approved a resolution to inform state pension officials that the amount of commercial development that will be permitted on the 320 acres in Alexandria and Arlington known as Potomac Yard probably will be substantially less than what is permitted now, making the land less valuable.

The land is owned by RF&P, formerly the Richmond, Fredericksburg and Potomac Railroad. Officials of the pension fund are considering buying a majority ownership in RF&P, which is planning a 17-million-square-foot development at Potomac Yard.

Potomac Yard, which is between Route 1 and the George Washington Parkway, has long been thought of as an investment that could yield substantial profits for its owners. The area, now used as a rail switching yard, is believed to be one of the largest undeveloped urban tracts on the East Coast.

Last week, three of the council's seven members, in response to strong public opposition to possible traffic from the project, unveiled a plan to limit development in the yard to less than a third of what developers have proposed. A fourth council member has endorsed that proposal, virtually assuring its passage.

Council member Lionel R. Hope (D) voted against the resolution, saying, "I get a little nervous when we use our power to persuade somebody not to buy" something. "They have a willing seller and a willing buyer. I just don't like the idea of a resolution in this regard." Council member William C. Cleveland (R) also voted against the resolution.