Vladimir Nekrasov, who owns the Arbat Prestige cosmetics chain, was arrested in Moscow last January on tax evasion charges along with Sergei Shnaider, an alleged mafia boss also known as Semyon Mogilevich.
Nekrasov wrote that the prison chief and the investigator in charge of his case told him: "Do you want to get out of here, Nekrasov? Testify against Shnaider, and you'll walk free tomorrow."
Nekrasov, 47, has maintained that Shnaider, 62, is not linked to Arbat Prestige, but said in the letter published in Russian media on Monday that he has come under strong pressure to provide false evidence against Shnaider. In particular, he said his custody has been extended several times without a valid reason.
In April, the Moscow City Court refused to release Shnaider despite the offer of a $2 million bail payment, equal to the back tax claim against Arbat Prestige.
Shnaider has been wanted by a number of countries, including the United States, for more than 15 years for alleged involvement in organized crime and corruption. Believed to control the largest Russian mafia syndicate in the world, he is suspected of being involved in arms dealing, drug trafficking, the sex trade and money laundering.
He served two jail terms in the Soviet Union in the 1970s for currency dealing offenses and fraud.
Russian prosecutors allege that Shnaider was behind a tax evasion scheme said to have been used by Arbat Prestige.
Arbat Prestige was established in 1989 and opened its first retail outlet in 1998. It now has a network of 64 outlets across Russia.
