The Federal Trade Commission backed off an antitrust case yesterday, saying it has agreed to a proposed settlement in its five-year fight over Borden Inc.'s pricing practices for its reconstituted-lemon-juice product.

Under the settlement, Borden would not be allowed to sell ReaLemon for less than the product's variable cost, which is affected by changes in production. A commission representative said the settlement provides Borden with specific ground rules to follow in pricing its product rather than a vague edict to avoid "unreasonably" low prices. She said the commission's philosophy has changed since its original order was issued five years ago.

Bordon had appealed a 1978 FTC order blocking the firm from selling the product at unreasonably low prices, and the appeal is now before the Supreme Court. The FTC said it will ask the court for a 60-day delay to permit public comment on the proposed settlement.

A 1974 FTC complaint charged that Borden illegally monopolized the reconstituted-lemon-juice market by using its power and size to restrain the growth of competitors. In 1978 the commission held that Bordon had accomplished that through predatory pricing--pricing ReaLemon at a low level with the intention of driving competitors out of the market.