Finance Minister Barend du Plessis said today that supporters of the U.S. House of Representatives vote to impose trade sanctions on South Africa were "making a grave mistake."

Many blacks cautiously welcomed the vote.

In a speech to the Cape Town Parliament, du Plessis said South Africa would easily be able to raise loans in Europe to replace capital from American banks.

Despite this expression of confidence, however, du Plessis warned South Africans not to take the disinvestment campaign lightly, suggesting that it might be part of a "diabolical onslaught to destabilize this region of Africa."

Business leaders displayed a similar dichotomy. Raymond Parsons, chief executive of the Association of Chambers of Commerce, issued a statement warning businessmen to start preparing their strategies to oppose the sanctions.

"The time has arrived for sharp realism," he said.

Then, in a switch of tone, Parsons suggested that South Africa might hit back at the United States by withholding vital strategic minerals.

Several government sources have suggested that South Africa should take retaliatory action.

Yesterday Deputy Foreign Minister D.J. Louis Nel said that if sanctions led to increased unemployment, South Africa might have to repatriate a million blacks working here to neighboring countries, a point taken up by several journalists in the progovernment Afrikaans-language newspapers. They have argued that it would be a way of demonstrating South Africa's pivotal role in the region.

Black reaction is divided. Many said they fear the effects disinvestment would have on their jobs, but most politically aware blacks privately spoke in favor of sanctions because they said South Africa's white-minority government will not move beyond cosmetic changes to its segregationist system, called apartheid, unless it is forced to do so by a combination of internal and external pressures.

Those who favor sanctions seldom say so openly because of uncertainty about whether it is a crime under South African law to do so.

Nor are all politically aware blacks agreed on the subject. Followers of Chief Gatsha Buthelezi, leader of the powerful Zulu tribe, have declared themselves opposed to sanctions because they say blacks would suffer the worst effects.

Followers of the relatively small black consciousness movement, while generally in favor of external pressures, are showing indifference toward the U.S. action because they are strongly anti-American.

It is in the large alliance of community organizations known as the United Democratic Front, which the government regards as a front for the underground African National Congress, that the most overt approval of the congressional vote is to be found.

An editorial today in the country's leading daily newspaper for blacks, The Sowetan, said: "There is no doubt that South Africans are going to suffer, yet we think blacks will suffer with a certain amount of grace, for after all the sacrifices will be made with the ultimate aim of freeing their children."