The Soviet Union has agreed to begin negotiations with the United States on a new long-term grain sales contract, responding to an offer by President Reagan to reopen talks that were suspended 16 months ago.

Soviet Ambassador Anatoliy Dobrynin telephoned Secretary of State George P. Shultz Monday night to accept the offer Reagan made last month, officials said yesterday.

At his news conference last night, the president said his decision will "restore something of what we lost with the embargo in the eyes of the world, restore us as being viewed as a dependable provider . . . I think the benefit will accrue to us, certainly, as much as to them."

The president stressed that any grain deal will require cash payments from the Soviets, who, he said, have extended their finances because of their recent military buildup.

"We are not offering any credit deals or anything of that kind. They are going to have to buy cash on the barrel head. And that is hard cash that they will have to come up with."

The president was asked if U.S. negotiations to sell grain to the Soviets conflict with American pressure on European nations to restrict trade with the Soviets. Disputes over East-West trade caused the last economic summit of Western allies to dissolve into a bitter dispute that prevented other issues from being settled. The administration has sought to keep East-West trade discussions off the agenda for this year's economic summit at Williamsburg next week.

"There is peace among us with regard to East-West trade," Reagan said. "And the only problems we had were subsidized credit and trade that was going on in which the Soviet Union was being allowed to purchase at below market value.

In a joint announcement yesterday of the Soviet agreement to begin negotiating a new grain deal, Agriculture Secretary John R. Block and U.S. Trade Representative William E. Brock said:

"President Reagan has continued to reaffirm our intention to be a reliable supplier. The Soviet Union's willingness to sit down and negotiate a new deal is a strong indication that this message is being heard. We are getting the pieces back together again."

No date has been set for the talks, but American and Soviet officials are scheduled to meet in London next month to discuss the existing agreement, which expires Sept. 30 after two one-year extensions.

That agreement calls for the Soviets to buy a minimum of 6 million metric tons of U.S. corn and wheat annually but allows them as much as 8 million metric tons.

No U.S. position for the bargaining has been set, but U.S. officials said yesterday they want to increase sales to the Soviets.

Reagan had ordered the suspension in talks on a new agreement because of the Soviet role in government repression in Poland. As recently as last July 30 the president said there would be no change in the existing grain pact because "the Soviets should not be afforded the additional security of a new long-term grain agreement as long as repression continues in Poland."

Senate Majority Leader Howard Baker Jr. (R-Tenn.), leaving the White House after congressional GOP leaders met with Reagan, applauded the president's offer to reopen the talks. "I think it is clear to everyone now that Poland is better off now than they were two years ago or a year ago, and I think this was an appropriate time to renew efforts to renew dependable trade relations with the Soviet Union," Baker said.

Rep. Kika de la Garza (D-Tex.), chairman of the House Agriculture Committee, said he hopes the administration will "press the Russians for an agreement which provides for higher minimum levels of Soviet purchases."

Sen. Robert Dole (R-Kan.) said: "With so many confusing signals being sent in other areas of our relations with the Russians, I am heartened that both sides have decided to get down to business on agricultural trade."

According to Wally Lindell, a spokesman for the Foreign Agricultural Service, the United States in 1979 supplied 78 percent of Soviet grain needs. After the Carter grain embargo of 1980 the amount dropped to about 30 percent.