The Soviet Union, three days after President Bush offered cut-rate prices on wheat, bought 300,000 metric tons of subsidized wheat and 200,000 tons of corn Friday - deals that broke the record for U.S. exports to the Kremlin.

With the sale, the Soviet Union has bought 19.3 million tons of American-grown corn and wheat since the trading year opened Oct. 1. The old record was 18.63 million tons set in 1984-1985, a year when the Soviet Union imported massive amounts of foreign grain.Bush's decision Tuesday to offer the subsidy ended a debate within the administration whether to sell wheat at a discount. A Cabinet council had not been able to settle the issue, leaving it to Bush to decide.

The Soviet sale issue had become entwined with questions over the future of the Export Enhancement Program, the major U.S. tool for battling export subsidies used by competitors in the world market. Some congressmen alleged the administration had suspended use of the subsidy program while it was under review.

The Bush administration has made two other large subsidy offers, both in late February, to China and Iraq.

Bush justified the subsidy offer to the Soviet Union as a way to maintain U.S. market share and to keep pressure on other nations to agree to world trade reform.

The wheat sold to the Soviets may be worth $48 million and the corn worth $22 million, based on prices at the Chicago Board of Trade. Five exporters were given subsidies worth $8.46 a ton on the wheat sale, or 23 cents a bushel. There are 36.7 bushels of wheat and 39.4 bushels of corn in a metric ton of 2,204.6 pounds.

Five months remain in the trade year. Exporters have sold 14.99 million tons of corn and 4.31 million tons of wheat. All of the wheat has been sold at a subsidy through EEP.

Analysts have projected large grain sales to the Soviets due to a decision to expand livestock production despite a disappointing 1988 grain harvest and stagnant production of oilseeds.