Randall Robinson's lament at the refusal of Congress to be more generous in providing aid to poverty-stricken Africa {Outlook, Oct. 5} conflicts oddly with his call for a ban on all American trade with and investment in South Africa, matched by the same mandatory sanctions by the U.N. Security Council.

As anyone familiar with the realities of regional interdependence knows, the first victims of such a total boycott would not be the economically strong and remarkably self-reliant apartheid regime, but the economically weak and vulnerable front-line states. Efforts to reduce their dependence notwithstanding, their trade, transportation and power links with South Africa remain a matter of survival. That is why even comparatively strong Zimbabwe has not found it possible to impose any sanctions that could provoke South African countermeasures.

Ending this dependence and revamping the region's South Africa-oriented infrastructure would require not only a vast increase of the present miserly congressional authorization of $44.5 million for southern Africa but a massive international effort that would take many years to complete. Whether such a politically motivated undertaking would make sense over the long run is debatable.

What is clear is that to push ahead with the kind of total South African trade and investment embargo Mr. Robinson advocates without first dealing with this problem would be the height of irresponsibility. But will Congress put its (our) money where its mouth is? Or does it prefer to treat the South African sanctions issue as the moral equivalent of the free lunch? HERMAN W. NICKEL Washington The writer was U.S. ambassador to South Africa in 1982-86.