The sight of one of the British embassy's senior security officials shuffling into a Kabul courtroom in leg chains and prison-issued pyjamas was not what UK diplomats had hoped for when they decided to pour money into Afghanistan's new anti-corruption tribunal.

Established as part of a drive to tackle Afghanistan's rampant corruption, the court was intended to target government ministers and public officials with the help of the UK, which is paying generous salary "top-ups" to its judges and prosecutors. But the defendant in one of the court's first cases is the British manager of the company responsible for guarding the British embassy in Kabul.

Bill Shaw, the commercial manager of G4S, and his co-defendant, an Afghan bodyguard, Maiwand Limar, are accused of paying a $25,000 (£16,500) bribe to secure the release of two confiscated armoured vehicles, a crime that carries a potential 10-year jail term. The company says Shaw made what he thought was a legal release payment to the authorities, for which he repeatedly tried to obtain a receipt from senior government officials.

The case is an embarrassing start for the court, which was established soon after the London conference on Afghanistan at which the government committed itself to tackling government graft.

One foreign legal expert involved in justice reform programmes said: "There is simply no way that such a high-profile, internationally sponsored court should be hearing a case about a man who paid $25,000 to get his car out. If you are going to have a high-profile court like this it should be focusing on public officials guilty of taking serious money."

The lawyer said the case – the fifth trial to be heard by the court – could reflect the political agenda of Hamid Karzai, the Afghan president who has consistently blamed foreigners and international contractors for much of the endemic corruption in his country.

Shaw smiled and cracked jokes with G4S colleagues when he arrived at the court on Tuesday, but his lawyer, Kimberley Motley, said he was deeply depressed after more than two weeks in a Kabul jail. Throughout the three-hour hearing, Shaw – a former royal military police officer who was awarded an MBE – remained silent and listened without a translator to proceedings conducted almost entirely in Dari.

He was arrested on 3 March, four months after the alleged offence took place on 31 October, when he is said to have arranged for a man called Eidi Mohammad, who he thought worked for the National Security Directorate (NDS), to be paid $25,000.

The NDS had confiscated the two armoured cars earlier in October for not having proper licence plates, although the company said it had done all it could and that the necessary documents were "pending". Shaw's lawyer said the NDS had since denied Mohammad worked for it at the time, and so Shaw was not guilty of any offence.

If that was the case, said the lead judge, Abdul Jalil Farooqi, the real victim was Shaw for being "cheated out of $25,000". But he refused a request to free Shaw and Limar, instead adjourning the case to allow the prosecution to collect more evidence. Mohammad, who investigators said has "escaped", has not been questioned by police.

The UK pays the rent for the court's new building, a mansion in the glitzy Sher Pur area of Kabul. The neighbourhood, which is often seen as a symbol of the country's endemic corruption, is a strange choice for a flagship new courthouse. Its garish houses are known locally as "poppy palaces" because of suspicions that some were built with drug money.

Despite the British financial support, Farooqi said his court lacked basic facilities such as filing cabinets. More seriously, he said the court also lacked security guards, which are seen by most justice experts as essential if judges are to risk prosecuting Afghanistan's most powerful criminals.

A spokesman for the British embassy said it was right that the UK government was sponsoring the work of the court and that consular staff were providing support to Shaw, including paying regular visits to him in prison. He said the embassy was aware of a "wider issue with vehicles being confiscated on tenuous grounds".