Russia will receive $70.7 million a passenger from NASA to train and transport another six crew members to the International Space Station (ISS) under the latest modification to its Soyuz transportation services contract with the U.S. space agency.

The six-seat add-on, which will run through 2017, comes as a showdown between Congress and the White House develops over how much funding NASA’s efforts to spur a commercial U.S. route to the ISS will get in the fiscal year that begins Oct. 1.

President Barack Obama’s fiscal 2014 budget request for NASA includes $821 million as the government’s share of the public-private Commercial Crew Program, while some powerful lawmakers with NASA oversight would like to see $300 million of that amount go to the heavy-lift Space Launch System (SLS) instead.

Administrator Charles Bolden and other top agency managers have been arguing for months that without full funding of the commercial-crew request, they will have to delay the planned inauguration of commercial crew flights to the station and continue to pay Russia for seats in the three-seat Soyuz capsules it sends to the station regularly.

“Because the funding for the president’s plan has been significantly reduced, we now won’t be able to support American launches until 2017,” Bolden stated in a blog posted under his name Tuesday, coinciding with the new Soyuz contract announcement. “Even this delayed availability will be in question if Congress does not fully support the president’s fiscal year 2014 request for our Commercial Crew Program, forcing us once again to extend our contract with the Russians. Further delays in our Commercial Crew Program and its impact on our human spaceflight program are unacceptable.”

The $424 million modification to NASA’s contract with the Russian Federal space Agency (Roscosmos) covers training, launch, flight operations and landing of six astronauts NASA will send to the station between mid-2016 and the end of 2017. The firm-fixed price modification also covers some support at the Baikonur Cosmodrome that previously was covered under a separate contract, NASA said.

Even so, the new contract modification marks another price increase for Soyuz seats. The previous modification, a two-year deal covering 12 seats that runs through mid-2017, totaled $753 million, or $62.75 million per seat. Under the contract that ends next year, NASA pays $55.8 million a seat, and before that it was paying $51 million per passenger, according to previous NASA announcements.

“NASA is committed to launching U.S. astronauts aboard domestic spacecraft as soon as possible,” Bolden stated in his blog post. “Full funding of the administration’s fiscal year 2014 budget request is critical to making these domestic capabilities possible by 2017.”

Rep. Frank Wolf (R-Va.), chairman of the House subcommittee that funds NASA, and other lawmakers have questioned the agency’s need to help fund three companies — Boeing, Sierra Nevada and SpaceX — in their commercial crew vehicle development. In Senate Appropriations testimony last week Bolden repeated the administration argument that at least two competing suppliers are needed to hold down prices over the long haul.

Bolden told the Senate panel that if sequestration extends into fiscal 2014, NASA will have to rewrite the $3.2 billion in contracts with SpaceX and Orbital Sciences Corp. to deliver cargo to the ISS in vehicles developed under a scheme similar to the Commercial Crew Program.