The seller, a South Korean businessman with a company specialised in the crypto-currency in Singapore, accepted the proposition nevertheless and met the Serbian buyer in July.
Hyper-wired South Korea has emerged as a hotbed of trading in virtual units.
Billionaire investor Warren Buffett on Monday said buyers of bitcoin, which he has characterised as "rat poison squared," thrive on the hope they'll find other people who will pay more for it.
Police busted a crime ring based in Spain that used credit cards and cryptocurrencies to launder €8mn ($9.8mn) from drug traffickers, Europol said on Monday.
Bitcoin is a virtual currency created from computer code that allows anonymous transactions and its value has soared since it came into being in 2009.
Bitcoin extended its sharp tumble of the past 24 hours, skidding more than seven percent on Wednesday in a rapid downturn in fortunes as investors were spooked by fears regulators might clamp down on an asset whose value has skyrocketed in the past year.
A series of measures have failed to curb overheated virtual currency speculation in South Korea and Justice Minister Park Sang-Ki said it would be "devastating if the bubble bursts".
Bitcoin plunged by a quarter to below $12,000 on Friday as investors dumped the cryptocurrency in manic trading after its blistering ascent to a peak close to $20,000 prompted warnings by experts of a bubble.
Bitcoin futures jumped more than 20 percent in their eagerly anticipated US debut, which backers hope will encourage wider use and legitimacy for the world's largest cryptocurrency even as critics warn of the risk of a bubble and price collapse.