The Conservatives have sold their old headquarters in Smith Square, Westminster, in a £30.5m deal.
The Tories have been trying to sell Smith Square for three years
The sale is reported to have made the party a profit of about £15m which it will use to pay off most of its debts.
Labour, by contrast, owes about £23m with lenders who bankrolled its election campaign demanding repayment.
The Tories said 32 Smith Square, and neighbouring flats in Tufton Street, had been sold to an undisclosed European company.
The Electoral Commission launched an investigation into the parties' property dealings last year amid allegations they could represent disguised donations by a foreign donor, breaching electoral rules.
But the Commission said earlier it was "satisfied" the sale of the Smith Square property was a commercial transaction, rather than a donation, after being shown independent valuation documents by the Conservatives.
A Tory spokesman said the party had signed a confidentiality agreement with the new owner at their request, but he said they were not party donors.
"There is nothing suspicious about it," he told the BBC News Website.
"It is just they have asked for their name not to be disclosed."
The Conservatives have been trying to sell their old head office since July 2004 when they moved to more modern premises in nearby Victoria Street.
The Smith Square building was the scene of many iconic moments in the party's history but its "rabbit warren" interior was also reputed to have been a breeding ground for plots and intrigue.
The party is due to move into new offices at 30 Millbank, on the banks of the Thames, when the Victoria Street lease runs out next month.
Labour occupied floors in the separate building of Millbank Tower from 1995 to 2002.
Contracts were exchanged with the new owner of 32 Smith Square on Wednesday and completion will take place next month.
In a statement, the party said the new owner was "a European company with wide experience in property investment and development".
"The company has other significant operations and assets in London, including high quality refurbishment projects," it added.
Cash is reported to have been flowing into Tory coffers since the election of David Cameron as leader.
The Spectator magazine claimed £21m had been raised during the last year, compared with £15m in a normal non-election financial year.
However, some of its biggest donors, including spread betting magnate Stuart Wheeler and retail tycoon Lord Kalms, have expressed disquiet at the direction in which Mr Cameron is taking the party.
Labour, meanwhile, still faces a £23 million-plus debt despite a £2m donation from Indian steel tycoon Lakshmi Mittal last month.
The party sold its old head office in Old Queen Street for £6.5m but that was spent on repaying loans to the Co-operative bank.
It is relying on financial support from trade unions and bank loans as it attempts to pay off debts to wealthy backers who bankrolled its general election campaign.