Funds managed by Equitix are buying the majority of Balfour's interests in five street lightling PFI projects.
Balfour said the cash consideration of £33m would be retained within the group, and said the proceeds from the sale were "in excess of directors' valuation".
The firm also said it may receive further cash consideration when the latest results of a number of the projects are determined.
News of the deal comes days after Balfour boss Leo Quinn said the group is on track to complete its turnaround.
The Build to Last transformation programme that Quinn implemented is aimed at cutting costs, adding cash to the balance sheet and improving governance and controls within the group.
"The actions that we have taken during the first two years of Build to Last have been necessary to lay a solid foundation for long term profitable growth," Quinn said earlier this week.
"Our people have responded to this challenge with passion and commitment. I am confident that the next 24 months of Build to Last will see the group achieving industry-standard margins."