Foundation Director Ivan Zhdanov told the television channel Dozhd on October 10 that a 140,000-ruble ($2,160) donation from a Spanish company was likely "a provocation" and not a real donation.

"Similar things have happened before; we have easily returned the payments, but it is quite difficult to trace a transfer made into a blocked account and it is impossible to return it even if we tried. I think it looks like a provocation," Zhdanov said.

Zhdanov spoke a day after Russia's Justice Ministry branded the nonprofit FBK a "foreign agent" under a 2012 law that was widely criticized by the West for stigmatizing groups with the designation.

Navalny has rejected the ministry's announcement, saying Russian authorities fear FBK because of the evidence of corruption it digs up on them.

Zhdanov said the FBK receives payments solely in rubles, and mainly through the payment system Yandex.Money.

He believes the Spanish donation took advantage of the system to try and set up FBK so the authorities could label it as a foreign agent.

Such a designation subjects the FBK to the same requirements as foreign-funded nongovernmental organizations, under which they must apply for inclusion in a government register and submit regular reports covering their sources of funding, their objectives, how they spend their money, and who their managers are.

"They have thought up a clever little scheme: a foreign organization with an account in rubles. They would not have succeeded if they had had an account in a foreign currency," Zhdanov said.

The Justice Ministry's document lists an individual named Roberto Fabio Monda Cardenas from Spain as the donor. Cardenas could not be reached for comment.

Navalny himself has said FBK has not received "a single penny" of foreign money and challenged the Justice Ministry to show any evidence to the contrary.

The Justice Ministry also listed a donation from an American company, Star-Doors.com LLC, as part of its reasoning for the foreign-agent designation. Zhdanov said that the FBK had not found any payments from Florida, which is where a company called Star-Doors.com is listed.

A man by the name of Yury who identified himself as the owner of the business but asked that his last name not be used, told RFE/RL he made a personal donation of $52.50 -- including a $2.50 transaction fee -- in June through his PayPal account to the account of Zhdanov, who was seeking a seat on the Moscow city council in elections in September.

Yury said he was seeking asylum in the United States but only possessed a Russian passport. "This was a transfer from one Russian citizen to another Russian citizen," he said of the donation to Zhdanov.

The Anti-Corruption Foundation was founded in 2011 by Navalny, Russia's most prominent opposition leader and a lawyer by training who has doggedly pursued evidence of corruption at the highest level of Russian politics.

Its investigations regularly provoke public uproar over misuse of state funds -- in 2017, a probe into Prime Minister Dmitry Medvedev's wealth became a catalyst for a wave of mass rallies that erupted across Russia that March.

Navalny recently served a 30-day sentence for allegedly organizing recent protests against the exclusion of independent candidates from the September elections to the Moscow city council. Of the 30 or so candidates barred from running, five were affiliated with or employed by the FBK.

In August, the foundation said all of its accounts had been blocked as the country's powerful Investigative Committee proceeded with a money-laundering probe that threatened its survival as a self-professed force for elite accountability and political change.

With reporting by Dozhd and Interfax