A Frederick businessman has signed a contract with the U.S. Postal Service that may end the two-decade dispute over the 65-year-old post office building in Frederick's downtown historic district.

John R. Laughlin Jr., president of Galaxy Conferences Inc., which arranges national conferences and meetings, signed a five-year lease-purchase contract that the postal service proposed July 1, a month after Mayor Ronald Young tried to buy the building for the city.

The city, which wants the building returned to its tax rolls, had planned to resell it. City officials support Laughlin's effort.

Laughlin said this week he offered to pay only $1,000 a month--not the $2,247.50 a month the postal service is asking--for the unrestored 1917 Greek Revival building, which the postal service contract would require him to rehabilitate. Postal officials said they will begin reviewing the Laughlin offer this week.

Last week, Young accused Postmaster General William F. Bolger of "playing games" with the city over the preservation of the old post office and called the most recent postal service offer "unfair, unsuitable . . . and expensive."

Young took a $122,000 city check--the amount of the building's appraised value--to the June 1 meeting with Bolger. But Bolger, who last fall had agreed to sell the old post office to the city, rejected an outright sale and said his staff would come up with a specific proposal.

In May, First Lady Nancy Reagan presented a historic preservation award to Young for his efforts in "saving" the old post office and other historic buildings in Frederick. The day following the awards ceremony, which was sponsored by the National Trust for Historic Preservation, the postal service advertised for bids to demolish the building.

The latest Bolger-Young meeting and the proposed contract apparently are outgrowths of the bad publicity the postal service received.

Laughlin said the contract includes an option for him to buy the building after five years for $122,000, if he has restored and maintained it to the postal service's satisfaction. City officials say it needs new plumbing, electrical and heating systems.

The postal service would retain the land around the building for use of the new postal facility, built beside it in 1977.