CONGRESS IS NOW completing its revision of the budget that President Carter presented last January, and the differences are interesting. The country's rising anxiety over inflation can be accurately read in the successive figures for the deficit. In Mr. Carter's original budget, the deficit for fiscal 1979 was to be $60.6 billion. That was a little higher than in the current year, a circumstance defended as unfortunately necessary to keep the economy expanding. Congress apprehensively shaved it down in the first budget resolution last May. Then came the California tax referendum, followed by the statistics showing that in late spring the inflation rate was back up over 10 per cent a year. Now Congress has firmly set a ceiling of $38.8 billion on the deficit for the year that begins Oct. 1. That's some $11 billion lower than the deficit for the year now ending, and the lowest since 1974.

Since the budget deficit is one of those numbers of larfe symbolic importance, perhaps it is necessary to note that this reduction does not represent much cutting of spending in the conventional sense. One reason for the lower deficit is the size of the coming year's tax cut. Congress does not intend to lighten the tax load as much as Mr. Carter had originally proposed. And then the congressional budget committees have also pulled down the deficit by reestimating current spending - taking account of the curious phenomenon known as underspending.

Particularly since the new budget procedure went into effect, with its legislated limits on spending, federal agencies have been careful to err on the high side in calculating their budgets. The result has been that, in the past several years, actual spending has run less than the budget by embarrassingly wide margins. Congress has now taken its clippers to these budget figures and trimmed then down to the amounts that are likely to be spent. It means that outlay and deficit totals have been dropped without, in most cases, any great effect on the programs. The most spectacular example is in defense. The president asked for $117.8 billion. Congress's budget provides $5.4 billion less than that - but the difference is money that would not really have been spent, or at least not spent in the coming year.

The congressional budget expands a couple of programs significantly. The money for farm subsidies is up by $2 billion from the president's original request for reasons not unrelated to the farmer's' protests here last winter. The education budget provides for the expanded aid to college students with which the administration is trying to fend off tuition tax credits.

But the president's national urban policy, brought out with great ceremony last March, has been reduced to a mere shadow. A few days ago we noted in this space that the final congressional agreement on the budget was being delayed by an impasse over the public-works funds for this urban policy. That dispute has now been resolved by a neat verbal formula that does not, to be candid, foreshadow any great outpouring of federal construction money for the cities.

Back to the deficit: Modern policy uses the federal budget as a great balance wheel, to maintain the stability of the American economy. The deficit that Mr. Carter proposed last winter was calculated to offset several dangerous drags on business and employment. One of those drags is a lagging rate of American exports that falls far short of paying for the country's imports. By committing the government to this lowr limit on the budget deficit, Congress makes it more urgent that ever for the country to balance its foreign-trade accounts.