As eyes widened in the line behind him, a well-dressed Cuban stepped up to the teller's window at the state-run bank here and opened a briefcase full of neatly stacked $100 bills.

After exchanging each stack for a pile of crisp, new 20 and 50-peso notes, the Cuban, a visting exile from Miami, lifted the bulging case and faced the incredulous onlookers.

"I have abigh faimly," he shrugged, "and lots of friends here."

The U.S. government estimates that as many as 70 per cent of all Cubans have relatives living in the United States. As travel restrictions loosen and Cuban makes an effort to reunite families long separated by the politics of revolution, hundreds of exiles, most with pockets and suitcases bulging, visit here monthly.

While the Cuban government prides itself on meeting the basic needs of most of its people for food, clothing and shelter, there is still a recognized desire for things Cuba either cannot or simply does not supply in quantities large enough to satisfy local demand.

Chief among them are medicines, leather shoes, stylish clothing and small aplliances-all of which are readily and cheaply available just 90 miles north of Cuba in the United States.

For most Cubans, it might as well be 90 million miles. Under the terms of a 17-year-old U.S. trade embargo, all direct U.S. commerce with Cuba ia prohibited.

Yet for the past 17 years, small amounts of U.S. goods and dollars have continued to reach the island through the exile community, and what originally was trickle of small packages sent by mail has turned into a comparative flood as more and more exiles temporarily return.

The government does not officially recognize the part these unofficial imports play in the Cuban economy. Some of Cuba's more hardline revolutionaries, moreover, refuse to accept goods sent by exiled relatives.

But Cuba has gone a long way toward facilitating the entrance of such goods through low or nonexistent duties, and through a growing enetwork of "diplomatic stores" that accomplish the dual purpose of circulating consumer items and bringing in much-needed hard currency.

Each of Cuba's large tourist hotels houses its own "diplo," as the stores are called. Two separate department stores in Havana will be augmented next spring when the government reopens-under state management-the old Sears Roebuck building here.

While some reisdent diplomats here patronize the stores, most of their sales are to the growing number of visiting former residents who buy up goods to distribute to their families.

AMONG the biggest-selling items are local products that are rationed due to scarcity and export potential, like Cuban coffee at $6.50 per pound. Many of the imported goods sold, like black-and-white televisions and $75 Japanese-made electric fans, are also manufactured in Cuba. But the government still cannot produce enough locally, or pay for enough imports, to meet the demand.

By selling only to those who can show a document proving they have traded hard currency for their pesos, the government gets back with profit the scare hard currency it must put out to purchase such imports in the first place.

While "diplo" prices are high, they are still generally lower than what Cubans here must pay for imported or even homemade "luxury" goods on the open market.

The government maintains that its goal eventually is to produce or import enough consumer goods so that every Cuban-with or without U.S. relatives, a high income, or high standing in his union or Communist Party orgainzation-can have equal access to television sets and electric fans.

That will mean increased industrialization and quality production, a primary goal of the current five-year economic plan and, ideally, a lifting of the U.S. embargo.

According to U.S. officials, lifting the embargo against Cuba is still very much a low-priority policy issue, awaiting Cuban movement toward with-drawing its troops from Africa and resolving private compensation claims for American businesses Cuba nationalized years ago.

While most of the trade prohibitions could be eliminated by simple executive order without congressional action, Castro himself has said he does not expect President Carter to take steps before the 1980 elections.

In the meantime, the "diplos" and exile dollars continue to supply at least a small part of the Cuban demand for blue jeans and rare treasures like crepe-soled shoes with leather uppers.