Officers are now recommending seeking borrowing the £20 million instead, which will cost the council £1.2m a year in interest for the next 20 years.
The North West Relief Road linking the A428 Harlestone Road with the A5199 Welford Road spent more than 30 years in the pipeline before it was finally granted planning consent in September 2020, despite opposition from local campaign groups.
It will eventually join up with the Northern Orbital Road, running to A43 near Moulton.
But only £32.5m of the total £54.5m cost of the 1.9km road is currently committed.
West Northamptonshire has already agreed to cough up almost £2 million on top of the money already in the kitty, leaving them still £20 million short.
A proposal in June saw the authority submit a bid to the Levelling Up Fund, a £4.8 billion government programme to support investment in infrastructure such as local transport projects.
But that failed so a meeting of the authority's Cabinet on Tuesday (December 21) will now decide whether to approve seeking a 20-year loan to allow the scheme to go ahead, even though it would cost almost half-a-million pounds in the 2022-23 financial year plus £1.2m every year thereafter in interest.
Other options include a massive redesign to reduce costs and scrapping the plans all together.
The North West Relief road was granted planning consent in September 2020 despite opposition from local campaign groups.
Phase one of the scheme, from the A428 to a roundabout near Grange Farm, just south of the railway line, is being constructed by the developers of Dallington Grange.
The first section has already been built as part of the Harlestone Gate development.
West Northamptonshire Council is responsible for building phase two, crossing the railway line and connecting the developer's road to the A5199, with work scheduled to start in early 2022.
The Northern Orbital Road joining the A5199 to the A43 has remained at the planning stage since the result of consultations in 2018.