The First Minister forgot to pack his trews for a trip to China in December 2011 - so he charged £259.40 for a new pair so he could be "appropriately attired" for a black tie ball.
Officials for the First Minister had tried to hush up the affair, refusing to comply with a Freedom of Information request until they were ordered to do so by the Information Commissioner.
Campaign group Taxpayer Scotland said: "He seems to have forgotten his trousers and his duty to taxpayers."
Scottish Liberal Democrat leader Willie Rennie said: "It appears as if the First Minister has been using his officials as a bureaucratic human shield to hide his failure to reimburse the government for his purchase."
Mr Salmond has previously faced criticism for spending up to £470,000 to visit last year's Ryder Cup.