Alan McAllister, 29, convinced Henry Docherty – who worked with the now Pope Benedict XVI in Rome – he needed work done to his gutters and drains.
However, the bogus worker did not do, or plan on doing, any work to the semi-detached house in Glasgow's Crookston area.
McAllister was caught when his DNA was found on paperwork left at Mgr Docherty's home and handed to police.
McAllister, of Cambuslang, pled guilty at Glasgow Sheriff Court to fraud between July and August 2010.
Passing sentence sheriff Ian Miller described his actions as a "despicable fraud" that was "preying on a vulnerable man".
He jailed McAllister for 320 days reduced from 360 days.
The court was told one of his former co-accused approached Mgr Docherty at his home, claiming to be from a builders' company. Procurator-fiscal depute Alan McArthur told the court the man suggested work needed to be carried out on the gutters and drains and it would cost £10,500. Mgr Docherty was under the impression Scottish Water would cover the costs but was told the work could be carried out and paid for and he could claim the cash back from the authority.
Mr McArthur said: "The complainer gave over £2000 cash and a cheque for the balance.
"The complainer then decided to contact Scottish Water himself who said they had no record of any builders contacting them about his property so he immediately cancelled the cheque for £8500.
"As a result of this contact with that authority he also contacted his insurance company who sent out an expert to look at the property and the man that came out said that these items the accused wanted to fix did not require any work and even if they did the maximum price would be about £3000."
The police were then contacted.
Mr McArthur added: "This accused's DNA was found on items of paperwork left by the builders at the locus."
Defence lawyer Charles Sharkey said his client had gambling debts. "He is also someone who has expressed regret, remorse and shame," he added.
Mgr Docherty, now 82, was the first British priest to serve in the Congregation of the Doctrine of Faith between 1979 and 1987, an organisation based in Vatican City that maintains Catholic orthodoxy.
During his time there he worked with the then Cardinal Ratzinger who was made Pope in April 2005.