The Post Office continued to threaten two sisters over a £34,000 shortfall even after a criminal trial was ruled out over faulty computer evidence, the BBC has learned.
A Glasgow prosecutor rejected the case in 2014 because of "issues" with the Horizon computer system.
However, the Post Office did not tell Jacquie El Kasaby and Rose Stewart the reason for the failed prosecution.
Instead they threatened to get debt collectors involved if they didn't pay.
In the end the sisters, who were fired from their jobs at Gorbals Post Office, agreed to pay £10,000 to settle the debt even though they had not taken the money.
Jacquie said she suffered serious mental health problems as she lost her business, her income and had to deal with the shame of being branded a thief despite being totally innocent.
Chartered accountant Bill Cleghorn, who was the sisters' adviser, told the BBC he would never have let them to settle the case if they had known about the failed prosecution.
The Post Office said it would not be appropriate to comment on the allegations.
This latest development comes in a new BBC Disclosure programme, Scotland's Post Office Scandal.
False accounting shortfalls caused by glitches in the Horizon computer system, which was developed by tech giants Fujitsu, have been headline news since the ITV drama Alan Bates V The Post Office screened at New Year.
More than 700 people across the UK may have been wrongly convicted on the basis of faulty Horizon evidence, about 60 of them in Scotland.
As well as those convicted, as many as 3,000 more former Post Office workers across the UK have been affected. Many lost their jobs and incomes, and some were ruined by having to repay false debts.
In England, the Post Office had the power to bring its own prosecutions but in Scotland they were brought by the Crown Office and Procurator Fiscal Service (COPFS) in the same way as all other criminal cases.
The lord advocate, Scotland's top law officer, said last month that COPFS was first told of issues with Horizon evidence in May 2013. Within months it told prosecutors to "carefully consider" cases where Horizon was a factor.
Jacquie and Rose's ordeal had begun the previous year, in November 2012.
When auditors turned up at the Gorbals Post Office, which they had run since 2006, Rose knew bad news was coming.
She had been disguising unexplained discrepancies in the system for months and had not told anyone.
Rose, now 58, said: "I just thought: 'This is going to sort itself out. It must.' There's no money going anywhere, so it must be in this system somewhere."
When the auditors told them the accounts were down by more than £34,000, Jacquie said she "almost slid down the wall".
The pair were immediately suspended and an investigation was launched.
Jacquie, now 55, said: "The seeds of shame were planted that day. And that's when my world, the life that I was living, that was changed that day."
Mark Daly investigates how the Post Office and Scottish justice system got it so badly wrong.
Jacquie had plunged her life savings into buying the Post Office. She and Rose had lost their incomes, and were in financial dire straits.
Rose sold her house to help the family with the debts.
Post Office investigators, whose bonuses were related to targets for clawing back losses, attempted to bring a criminal prosecution against the pair.
But in 2014, Glasgow procurator fiscal Angus Crawford assessed the case and told the Post Office investigators there would be no prosecution.
He cited "issues" with the Horizon computer evidence as the reason but the Post Office kept the decision secret from the family.
It then carried on pursuing them for the £34,000.
During a subsequent mediation process, the Post Office forced £10,000 from the sisters to settle the debt.
Their adviser told Disclosure the mediation process should have been told of the failed prosecution and the concerns over the Horizon computer system.
Mr Cleghorn said: "You were going in with maybe one hand tied behind your back, maybe both and possibly with a blindfold on as well because you were looking at something where you didn't have the full story."
He added that the withholding of such crucial information from a formal mediation process in order to extract money should be investigated.
Jacquie said: "It's appalling. I went into that mediation still feeling like the accused."
In a statement, the Post Office said it fully supported the aims of the ongoing public inquiry "to get to the truth of what happened" and that it would "not be appropriate" to comment on the allegations.
The BBC Disclosure programme also hears from the family of convicted postmistress Caren Lorimer, the latest case to be referred to the Appeal Court in Scotland.
Mrs Lorimer died in 2022, before she was able to clear her name.
She had worked in the New Farm Loch branch in Kilmarnock for 17 years before an audit in 2008 uncovered an apparent £38,000 shortfall.
Her husband, David, now 62, recalls Caren phoning him that day.
He said: "She couldn't believe what was happening. I don't think any of us could believe what was happening.
"I asked her if she'd done it. I shouldn't have, of course she didn't do it."
The police were involved, and interrogated Caren over the shortfalls. A transcript of the police interview reveals all the evidence had come from the Horizon system.
One detective put to her that the Horizon system was "completely and utterly fool-proof and [the auditors have] done all their checks and this money is not there. Nobody will ever believe you to sit there and say 'I put it in the system'. It's not there."
Caren and David's son was just four at the time. She couldn't bear the idea of prison and did what hundreds of other innocent postmasters did - pleaded guilty.
In 2009, she was sentenced to 300 hours community service, and forced to pay the Post Office £15,000.
Caren was diagnosed with cancer in November 2021 and died four months later.
"That's one of the hardest parts for the family," said Joanne Hughes, Caren's niece.
"She died basically a convicted criminal. No chance to clear her name."
Last year it emerged that when prosecutors in Scotland were told about the issues surrounding the Horizon system in May 2013 the Post Office sent a delegation of lawyers north to calm concerns.
Internal Post Office documents suggest this meeting "avoided unfortunate developments north of the border" which would have had "adverse publicity" in England.
To further reassure Crown Office officials, the Post Office hired Scottish law firm BTO to review all cases previously prosecuted in Scotland. The firm concluded all the previous cases were safe, needing no further review.
The Crown asked the Post Office for new expert reports to confirm Horizon's robustness. In the meantime it issued guidance that prosecutions could resume but that fiscals should be cautious.
In 2014, the Glasgow procurator fiscal turned down the Gorbals case.
The BBC can reveal that later that year, and despite the Post Office failing to produce the new expert reports, another Post Office worker was convicted in Scotland on Horizon evidence.
In 2015, with the Post Office unable to provide further evidence, the Crown Office formally stopped prosecuting Horizon cases.
For the next five years, despite having halted prosecutions due to concerns over the Horizon system, the Crown failed to review any of the 60 convictions already secured which involved Horizon evidence.
In 2019, 555 postmasters won a landmark victory against the Post Office in the English Civil Courts.
This seemed to spark the Scottish justice system into action and a year later, in 2020, the Scottish Criminal Cases Review Commission wrote to more than 70 people who might be victims of a miscarriage of justice, including Caren Lorimer, inviting them to apply to have their convictions quashed. Caren's appeal will be heard later this year.
In a statement, the Crown Office and Procurator Fiscal Service told the BBC it had done "all it reasonably could given the state of the confirmed information available" at that time.
It said the "true extent" of the failings was not known in 2015 and it was not until the decisions by the courts in England and Wales in 2019 and 2021 that the full extent of the bugs and errors became known.
It said that until that point the Post Office had maintained that the Horizon system was reliable and safe.
It added there had been "no scope for a meaningful review until the failings were known".