The UK's London Underground will have until 21 May to restate its upgrade charges for the Jubilee, Northern and Piccadilly lines.
Rail contractor Tube Lines will respond to the transit system's revised terms by 11 June, according to the project arbiter.
Tube Lines is expected to issue its final decision on charging and financing by 30 June.
The contractor signed a 30-year public-private partnership agreement with London Underground in December 2002 for the upgrade work.
Tube Lines is expected to charge £4.46bn ($6.9bn) for its services over the next seven-and-a-half years.
The project work involves renovation of stations, replacement or refurbishment of tracks, and the delivery of cars for trains.