Metro's subway, which has become increasingly popular with sports fans at RFK Stadium, will stay open late two Sundays this fall to take home Redskin football fans.

The subway usually closes at 6 p.m. on Sunday, but the Reskins, a good draw again, have been assigned two 4 p.m. Sunday starts. That means the games will be over after the subway's regular closing time.

The Metro board yesterday extended Sunday service on Sept. 28, when the Reskins play Seattle, and Nov. 2, when they play Minnesota.

The extra service will cost Metro about $25,000 each Sunday, but that will be partially offset by about $15,000 in revenue, officials said.

On another matter, Metro General Manager Richard S. Page announced yesterday that Metro's bus tokens, which are used mostly by riders in the District of Columbia, will increase in worth from 40 cents to 50 cents effective Sunday.

Sunday is the day that Metro's big fare increases takes effect. Metro stopped selling tokens at its sales outlets this week, but banks and other institutions that handle Metro tokens may still have some on sale for 40 cents. The same tokens will be worth 50 cents Sunday.

The 50-cent tokens will be accepted as full fare in the District of Columbia during midday, late at night and on weekends and holidays, when Metro charges its offpeak fares. During rush hour, when the bus fare in the city is 55 cents, the token plus a nickel will buy a ride. Tokens can be used in the suburbs, too, but they have never caught on with suburban riders, who pay different fares.

In other action yesterday, the Metro board approved an amendment to an agreement between the District of Columbia, Maryland and Virginia that will permit $323 million in new subway construction to go forward.

However, the final contracts involving the $323 million cannot be awarded until the Metro board and local governments have agreed to a construction program for the years following 1981. In this way, Maryland can prohibit Virginia from completing some Virginia Metro segments until Virginia has provided Maryland with guarantees that Virginia will help pay for Maryland sections of Metro.

The problem arises because, under the best of schedules, the Virginia segments of the 101-mile Metro system will be open for service some years before the final two Maryland segments.