Salsbury, 47, initially faced 25 charges of theft, false accounting and obtaining transfers of money between January 1 1999 and July 31 2002, while employed as a clerk to the William Parker School Foundation Trust.
But several charges were dropped by the prosecution during the two-week trial at Croydon Crown Court, and Salsbury was acquitted on all but one of the remainder.
A jury convicted him of adding £1,000 to the figure stated on a cheque last Thursday.
The solicitor, who pleaded not guilty to all charges, was given a year's conditional discharge and ordered to pay £300 in costs.
Judge Timothy Stow QC said: "I appreciate it is enough punishment for a solicitor to find himself a defendant on criminal charges and to be convicted after a fairly lengthy trial."
The judge added he accepted that Salsbury had not been involved in a deception. He reasoned Salsbury believed he had been entitled to the extra £1,000 he entered on the cheque, and said he would give him the benefit of the doubt.
Salsbury is a former senior partner at Funnell and Perring solicitors and a one-time deputy coroner for East Sussex. He has served as on-call solicitor for both Sussex and Kent Police, and was part of the defence team for former William Parker deputy head Sion Jenkins in his initial trial in 1998 for the murder of his 13-year-old foster daughter Billie-Jo. Jenkins was formally acquitted in February last year after a second re-trial failed to reach a verdict.
During Salsbury's trial, his barrister Miss Sarah Forshaw argued Salsbury was held in high regard by those who knew him and he had often given people his professional help without cost to them.
Salsbury, of Main Road, Icklesham, was initially charged with dishonestly obtaining more than £24,000 from the school trust, which was set up by the Rev William Parker in 1619 to fund the education of boys in the parish of All Saints and St Clements.
Its function is to use the income from various investments, as well as two properties in Priory Road. These sources generate an annual income of between £35,000 and £40,000.
The five trustees are believed to be liable for any losses incurred.
Dave Spillett, the chair of the William Parker School Foundation Trust, confirmed the trust was not awarded any compensation as part of Salsbury's sentence, but did not wish to comment about the trust's current financial situation.
He said: "The case has now been concluded, a verdict has now been arrived at, and it's now necessary for the trust to draw a line under this."