NOW THAT President Reagan and the congressional leaders have agreed to reduce the budget deficit, what comes next? There's an uneasy pause as they wait for a reaction. The stock market rose a little yesterday, but the dollar's exchange rates fell again. The unproductive struggle over the budget for the past month has demonstrated chiefly that the American government is unable to find either the will or the inspiration to make more than the most minor changes in its budget policy.

When you hear that the deficit is to be cut $30 billion this fiscal year, you need to ask: Cut from what level? The answer is that these cuts are calculated from the deficits that would otherwise occur, and those deficits are rising. The $30 billion cut this year, if it actually happens, will do little more than to hold the deficit to the same range as last year, when it was $148 billion. The $46 billion cut proposed for 1989 would be similar.

The message to the financial markets, and to anyone else who cares to listen, is that Mr. Reagan is leaving the present deficit to his successor and Congress is not strong enough to impose any alternative of its own. There was great progress in deficit reduction last year, much of it unexpected, but the present compromise warns the world not to expect much more until the next president has taken office and begun to work on his own budget.

But things might not work out so smoothly. The financial position of the United States is now extremely unstable, and the odds do not favor those politicians who are counting on positive thinking to get the economy through the next 14 months to the next inauguration. This country is consuming far more than it produces and is investing more than it saves. If it cannot begin to adjust and to balance its accounts by calculated policy, it runs a high risk that the markets will force it to adjust in their usual blind and brutal fashion. The stock market crash last month was the beginning of that adjustment process.

A recession next year is not a certainty. But the chances of it are greatly increased by the inability of the government to react quickly and surely to events like the crash. This adjustment, as it proceeds, will tend to push consumer spending down and interest rates up. It is impossible to forecast how fast this change will take place or who will get hurt. But it is apparently going to go forward in the absence of fiscal policy. Immobilized by the prospect of next year's elections, the people who make budget policy here are letting the economy take its own course. The direction in which it's turning is not reassurin