The trustees of financially troubled American University decided yesterday to delay employe pay raises for at least six months and to tie any raises after that to the number of students who enroll in the fall.

At the same time, the trustees approved a 12.9 percent increase in tuition, which will reach $7,000 next year.

Officials of the American Association of University Professors said AU was the only sizable university they know of to link pay raises to enrollment, which accounts for most of the college's revenue. However, workers in several industries have agreed to tie wages to future profits to help save jobs in struggling companies.

"We feel this type of arrangement is contrary to our principles," said Iris Molotsky, public information director of the AAUP, the nation's major college faculty group. "We would be very uneasy if it became a general practice. On the other hand, we recognize these are difficult times and this a way to retrench without layoffs."

"We will just wait and see how the dollars come in," said American University President Richard Berendzen. "We felt the best part of wisdom is not to commit funds until you are categorically sure you will have them.

"What we certainly won't do is do anything to lower standards just to increase enrollments," Berendzen said. "Our admission standards will stand still or they may even go up." He said he was urging faculty members to make special efforts to recruit good students.

Berendzen said the trustees had decided to postpone regularly scheduled raises from July 1 at least to next January.

If enrollment next fall decreases less than 6 percent, he said, the university would consider raises "up to 4 percent," with the size of the actual raises depending on the size of the student body.

If enrollment falls 6 percent or more, there will be no raise next year.

Earlier, Provost Milton Greenberg said university officials wanted to avoid a repetition of the layoffs and other budget cuts that took place suddenly last fall when enrollment fell an unexpected 5 percent.

Berendzen said he and the trustees rejected making any cuts in regular faculty positions or any more cuts in nonteaching staff because "we don't want to destroy quality and programs."

American University now has about 11,500 students, a full-time faculty of 470, and about 830 nonteaching employes. Last year tuition accounted for about 95 percent of its teaching and research budget, compared to a national average of 65 percent for all private universities.

Last month proposals by university administrators for a pay freeze were opposed by faculty and other groups. However, Bruce Norton, chairman of the University Senate, said yesterday that faculty members "would be willing to make the sacrifice, though some will do it grudgingly."

Norton urged the trustees to avoid any "frivolous" expenditures and to give "catch-up increases" in future years.

The $800 tuition increase was endorsed by the university's student government as necessary to avoid program cuts even though it came after an 18 percent increase last fall. The trustees promised financial aid would be raised by about $910,000 to match the tuition boost plus $200,000 more to make up for possible cuts in federal student aid.

With a 9 percent increase in dormitory rates and anticipated expected rises in board and other expenses, the total cost for a year at American University is expected to reach about $12,200 next fall. It already costs students more than $13,000 a year to attend the highest priced private universities, such as Harvard and Stanford.

Over the past three years salaries at AU have climbed by 35 percent, according to Jody Goulden, AU's public information director, and now average about $37,600 a year for professors. The previous pay raises have been considerably greater than inflation, she said. The national rate of inflation has slowed to about 4 percent a year.