John F. Herrity, chairman of the Fairfax County Board of Supervisors, made out a check for $61.18 yesterday and gave it to the county, fulfilling his pledge not to accept more than a 7 percent salary increase this year.

The $61.18 was the two-week difference between a 7 percent increase raise and the 23 percent raise that the Board of Supervisors voted last April. The raise, increasing pay of the county's nine supervisors from $15,000 a year to $18,455, became effective this month.

Supervisor Audrey Moore (D-Annandale), who also had promised she would accept only a 7 percent raise, said she was returning her first check of 1980 because the county did not deduct the difference as she had requested.

Moore was the only supervisor who voted against the 23 percent raise last April. Herrity, along with the other supervisors, said he voted for the increase on the assumption it was within the president's wage guideline because the supervisor's salaries had not increased in four years.

Herrity changed his mind and promised to return the difference after the federal Council on Wages and Price Stability said the 23 percent raise was not within the federal guideline, but did not oppose it.

With their self-imposed cuts, Herrity and Moore will make $16,050 a year.