A Fairfax County judge fined Washington Redskins' defensive end Dexter K. Manley $100 yesterday for altering his car's temporary license tags and imposed a conditionally suspended six-month jail sentence and $600 fine for impersonating a police officer.

Manley pleaded guilty to both misdemeanor charges in General District Court in a plea-bargain agreement with prosecutors.

The charges stemmed from a Nov. 23 incident in which a Fairfax County police officer stopped Manley, 23, in Herndon after noticing the temporary tags on Manley's 1981 Mercedes-Benz 450 SEL appeared to have been altered.

As the officer was questioning Manley, he noticed a Fairfax sheriff's deputy's badge pinned inside Manley's wallet. Manley reportedly told the officer he was a deputy sheriff.

Police said they contacted the sheriff's department and learned the football player had worked as a deputy a few months during the off-season, but had resigned in June.

Police incorrectly arrested Manley on a felony charge for allegedly altering the license tags, according to Chanda Myers, assistant Commonwealth's Attorney. Virginia law was changed July 1, reducing that offense to a misdemeanor. The charges were changed yesterday and Judge Frank B. Perry III fined Manley $100 for failing to obtain permanent registration for the automobile.

Perry also imposed a six-month jail term and $600 fine on Manley for impersonating a police officer.

Perry suspended the sentence and the fine on condition of Manley's "good behavior for one year."

Manley's attorneys said earlier that the football player, a Houston native, had ordered auto plates from Texas where the car was purchased, but that those tags had not yet arrived.