The Agriculture Department is weighing a request from the Soviet Union to buy up to 500,000 tons of soybean oil at subsidized prices. If the deal is approved, it would be the first time the United States has subsidized the sale of a farm commodity other than wheat to the Soviets. But the proposed deal is caught up in a web of complex foreign policy and economic issues, sources told the Washington Post. Soybean farmers and processors have written President Bush urging that the Administration make the sale. But Agriculture Secretary Clayton K. Yeutter has warned that the sale could hurt U.S. ties with countries such as the Philippines and Malaysia, whose exports of vegetable oils could suffer.