The tone in Washington this week toward its Central American problem was markedly different. The overtones were cheerful rather than anxious. Merit was seen in assertions from both Cuba and Nicaragua of readiness to negotiate. President Reagan's special emissary for Central America, Richard B. Stone, met and talked with a representative of the rebels of El Salvador.
Plans went right ahead for six months of a major United States military presence in Central America, but the new implication is that all of this will lead to a negotiated resolution rather than serious shooting.
No one can prove a connection between this change in tone toward Central America and the announcement on July 28 that the Soviet Union had agreed to increase its purchases of US grain by 50 percent.
Yet it is a fact that the Soviets did something helpful to President Reagan by agreeing to take at least 9 million metric tons of US grain off his hands for each of the next five years.
The Soviets are currently buying US grain at the rate of only 6 million tons a year, although they have been offered 23 million tons.
Moscow does not do a favor for an American president without expecting something in return. What is the quid pro quo? No official is saying. But there is clearly a change in the tone of US relations with the Soviets. The hostility that was there prominently during the first two Reagan years is draining away. It has been replaced by a sense of ''business as usual'' between the two superpowers.
Arms control talks have been recessed for a summer holiday, but the expectation on both sides seems to be that progress is being made, that both sides are moving cautiously to some form of new agreement within the year.
It is known that US Secretary of State George Shultz is in frequent conversation with Moscow's US Ambassador Anatoly Dobrynin, and will probably have another chat soon with Soviet Foreign Minister Andrei Gromyko.
There are still voices in Washington, principally at the Defense Department, arguing against any deals or even negotiations with the Soviets. But those voices, although still strong and still with influence, seem to be losing out to those who are ready to get back to ''normal'' relations with Moscow.
''Normal'' in this case seems to mean that the effort to influence Soviet policy by a campaign of economic sanctions has run its course, has achieved little except to put a strain on US relations with its NATO allies, and might as well be dropped.
Part of this changed relationship would require a new understanding about the leftist movements in Central America which have greatly worried Mr. Reagan.
The Soviet interest in Central America is that Cuba and Nicaragua - both economic and military clients of the Soviet Union - be allowed to continue to be Marxist in their internal policies and friendly to Moscow in their foreign associations.
The prime interest of Mr. Reagan is that there be no further export of Marxism in the areas from Cuba and Nicaragua, and no more Soviet clients in the area.
Mr. Reagan has made his interest, and his intent, clear by his program for military deployment of naval and ground forces into the area over the next six months. He has this week also indicated his readiness to negotiate with both Cuba and Nicaragua, meaning a willingness to allow them to continue to be Marxist and clients of Moscow, in return, of course, for an end to export activities.
He is even in contact, through Mr. Stone, with the rebels in El Salvador. That means that he can conceive of a negotiated end to the civil war there in which the political elements behind the rebellion have some share in a future government.
The change in tone toward the Central American affair could be only an attempt to calm down Congress and the public, which have become alarmed by three mighty ''battle forces'' heading for Central America and by US military exercises in Honduras to last several months.
The change in tone could be only that - were it not for the coincidence of the new grain deal, which brings the Soviet factor into the picture.
Why did both Cuba and Nicaragua declare their willingness to negotiate with President Reagan?
The only reason that makes sense is that they have been informed by Moscow that they can expect no help if they get themselves into shooting trouble with Mr. Reagan. Hence, they had better talk their way out of the confrontation that is coming up in Central America.
The grain deal is the obvious way of making this condition manifest. The Kremlin would not logically be basing its economic plans over the next five years on heavy imports of US grain if it were willing to have a real tussle with Washington over Cuba and Nicaragua. And Washington would hardly be willing to sell the grain if it expected a physical Soviet role in Central America.
There has to have been a tacit deal in order to explain all that has been happening over recent days and weeks.
The only tacit deal that makes sense is that Moscow gets to keep Cuba and Nicaragua in its client camp in return for a clear understanding that that is the limit. El Salvador is to be pacified on a status quo basis. Honduras and Guatemala are to be left in the US camp.
This is the kind of tacit deal that is to be expected between two rival superpowers which expect and intend to live with each other on a basis of competitive coexistence rather than on a basis of confrontation and possible ultimate war. It does not mean, or need not mean, going back to the kind of detente Henry Kissinger once set up between Moscow and Washington. That one was so chummy that the rest of the world grew uneasy. There were overtones of a sort of US-Soviet joint management of the world.
That kind of relationship is inconceivable with Mr. Reagan in the White House. But it is clear by now that Mr. Reagan has abandoned his original policy of economic and political hostility toward the Soviet Union and is moving into a replay of that condition which, in the Eisenhower-Khrushchev era, was known as ''competitive coexistence.'' It involved doing such business of a mutually beneficial nature as was possible.
The grain deal is mutually beneficial.