Even as the Fairfax County Board approved a deal to divide the price tag for Dulles Rail on Tuesday, its chairman warned that the whole project could be in jeopardy if Congress and President Obama do not speed up their slow-moving debt talks. The congressional talks to cut federal spending and raise the debt ceiling by Aug. 2 could have a “domino effect” that would leading to the slashing of Fairfax County’s bond rating and jeopardize the Metro extension to Washington Dulles International Airport, according to a draft of a Tuesday letter written by Fairfax Board of Supervisors Chairman Sharon Bulova to Congressman Jim Moran.