North Korea is accused of counterfeiting US currency
In February North Korea agreed to a timeline for giving up its main nuclear site by April - in return for badly-needed fuel and the return of $25m from a bank in Macau.
But the money has yet to be transferred, and the site remains open.
Now, though, the Russians are offering to step in and get the money to Pyongyang, which could remove a key sticking point in neutralising North Korea's nuclear capability.
How has such a small amount of money become such a sticking point?
The money is, in a sense, only the visible part of a broader problem.
Dozens of North Korean government departments do their international business through a bank in Macau called Banco Delta Asia.
But in September 2005, the US Treasury accused BDA of being a conduit for laundering money for Pyongyang, triggering severe limits on the bank's dealings with US financial institutions - and a freeze on $25m of North Korean money in the bank's accounts.
According to the Treasury, BDA was a "willing pawn" of North Korea, helping process as much as $500m a year in dirty money without asking awkward questions.
The move sent relations between Washington, DC and Pyongyang - frosty at the best of times - into the deep freeze.
Between then and now, the $25m became a tool for the US to achieve a deal on North Korea's ambitions for nuclear weapons - and an excuse for North Korea to stall.
So has a deal been reached now?
Yes, in February this year. North Korea pledged to give up its nuclear reprocessing activities in exchange for thousands of tonnes of fuel.
At the same time, the $25m would be unfrozen, and could - in theory - head back to North Korea.
But the money has yet to leave Macau, because at the same time the US Treasury cut BDA off altogether from the US banking system.
Unblocking the BDA accounts is key to progress, the North has said
This ultimate sanction, in banking terms, was made under section 311 of the USA Patriot Act - passed shortly after the 11 September 2001 attacks on New York. Effectively, it bars any financial institution from having anything to do with BDA, if they want to do business with or in America.
Understandably, therefore, attempts to find a way of wiring the money back to North Korea have failed.
Banks in China and Vietnam have been approached and have refused to get involved.
One US bank - Wachovia - has been asked by the US State Department to consider helping out, but it points out that it will need assurances that it is not in breach of section 311 before it can do anything.
What is Russia offering to do?
Russia is one of the partners in the six-way talks over denuclearisation of North Korea, and - given that it shares a border with North Korea - has a powerful interest in moving discussions along.
Early in June, Russian officials suggested that a Russian bank might step in to get the frozen $25m from Macau to Pyongyang - directly or via intermediaries.
The US Treasury has now acknowledged the possibility of Russian assistance.
But Russia is likely to require cast-iron assurances that US sanctions against banks which carry out transactions with North Korea will not apply.
One possibility would be for the money to go first via the New York branch of the US central bank, the Federal Reserve, and then on to Russia's own central bank before being paid into Moscow's Far East Commercial Bank, where North Korea has a long-unused account.
But what does a country like North Korea need money-laundering services for?
North Korea, in practical terms, is flat broke.
Its trade is minimal, its agriculture is suffering, and its contact with the outside world is severely limited.
But according to the US Treasury, not to mention many experts elsewhere in intelligence and financial crime, Pyongyang has for the past two decades made up for its lack of legitimate trade by taking an unhealthy interest in faking US banknotes, smuggling counterfeit tobacco products and even the narcotics trade.
Much of the proceeds, the US claims, have been laundered through BDA, which has also facilitated huge bulk cash shipments back to Pyongyang - as well as large trades in precious metals.
BDA, it should be said, has always strongly denied the allegations, insisting its business with North Korea is above board.
Why does the US not just send the money back itself?
In theory, the US could have provided a bank with the reassurance it needs to get involved, although that has yet to happen - and could, in any case, be legally tricky. "Difficult, yes; impossible, no," was how the State Department's spokesman described it.
The Yongbyon reactor is still operating
Similarly, reports have suggested that since 2001 there has been a conduit for fund transfers between the State Department's credit union and the Foreign Trade Bank in Pyongyang.
But as far as the Treasury is concerned, the ball is now in Macau's court. It is up to the regulators there to work out how to get the money back to North Korea - and in the meantime the section 311 rule stays in force.
In any case, after years of playing hardball with North Korea, the last thing the current US administration wants is look as if it is doing things Pyongyang's way.
Can't North Korea get it back any other way?
It could - for instance, through a direct withdrawal from BDA back to Pyongyang.
Alternatively, if the US is right that bulk cash shipments have been going on for years illicitly, perhaps the technique could be used for above-board purposes.
But as far as North Korea is concerned, that kind of deal is unacceptable.
It seems that the authorities in Pyongyang want the transfer to pass through the international financial system, so as to send a signal that handling North Korean money does not mean instant ostracism.
Not only that; keeping the matter rumbling on means more time to extract concessions - and, some experts fear, to keep reprocessing nuclear material.