WHEN CAN a president not spend money Congress appropriates? The question was last important in the early 1970s when President Nixon claimed the right to impound appropriated funds. It led to the Budget Control and Impoundment Act of 1974, the basis of the present congressional budget process. Now the issue has arisen again because the courts have disturbed the balancing mechanism in that act between the president and Congress.

The act created two methods by which presidents could avoid expenditure of funds -- rescission and deferral. Rescissions are outright cancellations of appropriations and the harder to achieve. Congress must formally undo what it has often just done months before. Both houses have to pass a rescission and send it to the president for his approval, just as with ordinary legislation.

Deferrals are different. In theory they have to do merely with the timing of expenditures; the appropriations stay on the books. The idea is to give the president flexibility to alter spending plans without a lot of fuss, and still preserve congressional control. The president must notify Congress of all proposed deferrals. Under the 1974 act, they were then to take effect unless either house objected.

But this one-house veto provision has now been upset by the Supreme Court's 1983 Chadha decision, which basically said no act of Congress is valid unless passed by both houses and offered to the president for veto, as provided in the Constitution. At the same time the president, as part of his effort to reduce the deficit through domestic spending cuts, is making aggressive use of the deferral power.

Three months ago Congress approved and he signed a bill appropriating $14.8 billion for the Department of Housing and Urban Development this fiscal year. Now he would shelve about half of that. Most of the appropriation is for subsidized housing and community development block grants. He wants Congress to rescind about $4.6 billion altogether and let him defer about $3 billion until next year. His view is that the deferral would reduce the need for a 1987 appropriation.

The president's position is that, to prevent the deferral under Chadha, both houses of Congress must now pass a resolution of disapproval (and have enough votes to override a veto). Several members of Congress, the League of Cities and Conference of Mayors and others have taken him to court, on grounds he is invoking a power he no longer has. They argue that, when the Supreme Court struck down the one-house veto, it struck down the entire deferral process, because the two are inseparable. The opposite ruling, they observe, would give the president the very line-item veto power that he has so often asked for, and Congress has refused to grant him.

The plaintiffs are right, of course. The president, who has tried for the longest time to cut back the housing programs and now badly needs the money to meet his deficit targets, has found a chink in the law and is trying to exploit it. He can say he is within his rights. But this is gamesmanship, and it is the wrong way to set important policy.