Always read the small print.

It’s a rule most of know and most of us ignore – and one often thrown back at borrowers by banks facing complaints about their terms and conditions.

Dmitry Argarkov, a Russian who was sent an unsolicited letter offering him a credit card, has turned the tables, however, and struck a blow for bank customers everywhere.

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Instead of simply ignoring the offer of credit, Mr Argarkov, 42, scanned the agreement into his computer, changed the terms of the contract and returned it to the lender, Tinkoff Credit Systems. According to the Russia Today news channel, his version of the agreement was rather more favourable than that he received from the bank. It had an unlimited line of credit, no fees and a 0 per cent interest rate.

He also added a provision stating that the customer “is not obliged to pay any fees and charges imposed by bank tariffs”, and an extra clause to the contract should the bank seek to break or change the agreement.

Crucially, when the document was returned to the Russian credit card provider, it failed to follow its own instructions and did not check the small print. Tinkoff Credit Systems sent him his credit card, which came with a credit limit large enough for Mr Argarkov to purchase a small island.

“The bank confirmed its agreement to the client’s terms and sent him a credit card and a copy of the approved application form,” Mr Argarkov’s lawyer, Dmitry Mikhalevich, told the Russian newspaper Kommersant. “The opened credit line was unlimited. He could afford to buy an island somewhere in Malaysia, and the bank would have to pay for it by law.”

Mr Agarkov, from the western city of Voronezh, also changed an internet address included in the document relating to the bank’s website, where the terms and conditions of the contract could be viewed.

After two years of use, the bank attempted to terminate Mr Agarkov’s credit card in 2010 because he was late paying the minimum required amounts. It tried to sue Mr Argakov for 45,000 rubles (£880), which was made up of the remaining balance of the account, along with fees and late-payment charges that were not in his altered version of the contract.

Earlier this week, a Russian court backed Mr Argarkov and ordered him to pay only the outstanding balance of 19,000 rubles (£371). “They [the bank] signed the documents without looking. They said what usually their borrowers say in court: ‘We have not read it’,” said the judge.

Not content with winning his day in court, Mr Argarkov is now taking matters further and trying to sue Tinkoff Credit Systems for 24 million rubles (£470,000) over its failure to honour the contract he created. For its part, the bank is counter-suing Mr Argakov for alleged fraud.

Oleg Tinkov, the founder of Tinkoff Credit Systems, tweeted: “Our lawyers think he is going to get not 24 million rubles but four years in prison for fraud. Now it’s a matter of principle. We don’t have small print, everything is clear and transparent. Try to open a card – then we’ll talk. Stealing is a sin. Not all in Russia think so.”

The court is set to review Mr Argakov’s case next month.