An American publisher announced today a 10-year agreement to publish Soviet technical works in English that he said could mean up to $87 million in U.S. sales and guaranteed at least $8 million in royalties to the Soviet Union.

Martin E. Tash, President of Plenum Publishing Corp. of New York and Washington, a technical printing house, said the 1980-90 pact "is the largest journal agreement of its kind ever concluded anywhere." Plenum will translate and publish material from more than 90 Soviet technical journals and periodicals.

The agreement, which Tash said took two years to negotiate, was concluded at the second Moscow international book fair today between Plenum and Vaap, the Soviet copyright agency. More than 500 foreign contracts have been signed with foreign houses, the Soviets have said with apparent pride.

Meanwhile, the sparring over free speech between the Soviets and major American publishers which has marked the fair amid the din of cash registers continued today. The Soviet press agency Tass criticized a buffet reception for Soviet writers hosted by the Association of American Publishers, whose invitation list included a heavy majority of authors whose works have been forbidden or sharply restricted here.

"It is surprising that among others, invitations were extended to people who have long compromised themselves by improper behavior," said Tass.

Among the writers who attended were Nobel Peace Prize laureate Andrei Sakharov, critic-memoirist Lev Kopelev, humorist Vladimir Voinovich, Marxist historian Roy Medvedev, and more than a dozen writers whose contributions to the "Metropol" collection which challenged Soviet censorship has brought on official reprisal.