The Scottish Government had withheld the letter to Stagecoach founder Sir Brian Souter on three occasions following requests from Labour.
The Herald has now obtained the document under Freedom of Information legislation (FoI). In it, the First Minister offered his "warmest congratulations" to Sir Brian, who has given £1.2 million to the SNP since 2007, for his controversial knighthood in the Queen's Birthday Honours last year.
Mr Salmond also praised Sir Brian's "leadership and outstanding philanthropy",
and his work tackling poverty globally. He added: "My personal congratulations on this well-deserved honour."
The knighthood was for services to transport and the voluntary sector and Sir Brian's Souter Charitable Trust, which has spent £32m relieving poverty since 1992.
After the nomination emerged, Labour MP Cathy Jamieson called for the SNP to look carefully at its relationship with wealthy individuals. Gay rights activists demanded Sir Brian's honour be revoked because of his role in a campaign to repeal legislation concering the discussion of homosexuality in schools in 2000.
Mr Salmond was cleared of helping to obtain the honour because an independent Honours Committee exists within the Scottish Government.
In April, The Herald had requested all correspondence between Sir Brian and the First Minister since 2009 to be made public under FoI laws.
Labour used the legislation to ask for all correspondence between the two men since 2007. The Government refused on the grounds information relating to honours is exempt under FoI, even though Labour's request had not mentioned honours. When Labour challenged this, the Government refused again, claiming the request was too vague.
Labour successfully appealed to the Scottish Information Commissioner, but the Government still refused to release it, on cost grounds.
Glasgow MSP Paul Martin, Labour's business manager at Holyrood, said: "This is a government that just can't be open and straight with people, even when it comes to covering up fawning letters to the First Minister's rich friends. Every week, we see more and more examples of the culture of secrecy operating at the heart of the SNP Government."
A Government spokesman said: "These claims are completely untrue because under this Government, the FoI process has improved significantly."