A family who sold their house to a firm so they could rent it back say a "typing error" has cost them £50,000.
Debra and Nigel Ford said the valuation on their four-bedroom cottage in Abercrave, near Swansea, was £195,000.
But before the deal went through, the firm buying it said the mistake meant property was worth only £145,000. They are still waiting for their money.
The BBC has been unable to contact the firm involved, Home Assured, despite repeated attempts.
Home Assured later defaulted on the mortgage and the house was repossessed.
The Fords said they felt they had to go ahead with the sale because the firm told them they would still have to pay £17,000 in charges.
The couple were looking to move from their Swansea Valley property when they responded to an advert in a Sunday newspaper.
Mrs Ford, 49, who is epileptic and has high blood pressure, said: "They said you could either sell and rent back or just sell. So we decided we'd sell and rent back for 12 months.
The firm said a typing error meant the valuation of £195,000 was incorrect
"They sent a surveyor. I didn't have access to the valuation or the report. They phoned me up three or four weeks later."
She said the man who rang said the house had been valued at £195,000.
She said: "I started to get a little bit concerned because they had told us initially that it would take only two to three weeks to go through but when we were about six weeks down the line I had another phone call to tell me that there had been a mistake and the house wasn't valued at £195,000, it was valued at £145,000."
The caller said there had been a typing error, said Mrs Ford.
Mrs Ford said: "I said that's impossible. How can it be a typing error or mistake. I said, look, forget it.
"I phoned the solicitor they had allocated to us and I told her we're not going to continue, forget about it, we haven't got to sell, we'll hang on." She said 10 minutes later, the man who rang earlier rang back to say the couple had clocked up £17,000 in fees which they would still have to pay if they pulled out.
She said: "I didn't know what to do. I phoned my husband. We didn't have that kind of money. He said we're just going to have to do it. That's what we did."
Mr and Mrs Ford sold their four-bedroom cottage in Abercrave
The next shock for the family was the rent. The house's new owners wanted £700 per month. When the property had been valued at the higher price, the rentable value was put at £1,200 per month.
She said: "I said I would rather move out. I didn't feel it was fair that they were asking me to pay them £700 rent when they had £27,000 fees and a £2,000 "maintenance bond".
"They withheld £21,750 which they called a security deposit, so that if we chose to re-buy the house within 12-months, that would be our deposit.
"The value of the house kept going down. We were expecting to have about £100,000 back. When it went down to £145,000 we were expecting to have about £50,000 back. But by the time they took their fees, we were left with just over £26,000."
The family moved out in January this year and rented privately at first but with no sign of their money, they went to stay with Mrs Ford's parents for five weeks.
They have since moved into the two-bedroom council flat in the Gendros area of Swansea.
Mrs Ford said: "I feel sick to the stomach. We've gone from a four double-bedroom cottage with a garden to living in a fourth-four flat that's absolutely soaking wet, freezing cold and you have no privacy whatsoever. And apart from that, I've had to get rid of three-quarters of my belongings.
"The thing that really gets me more than anything, is my little grandson. How can you tell a child that's so used to be playing in a huge garden there's nowhere for him to drive his dumper truck because we just haven't got anywhere to keep them?"
The couple are now living in this council flat
The family's case has been featured on BBC Radio 4's You And Yours programme.
The BBC has tried on a number of occasions to contact Home Assured.
John Pritchard, policy manager at Shelter Cymru said: "We advise people to be very wary of entering into sale and lease back schemes with private companies.
"These companies are currently unregulated, often pay the household well below the value of the house and families will have very little security of tenure as tenants.
"In the current recession, these companies could also have problems repaying their mortgages and this will leave households vulnerable to homelessness.
"If people get into difficulties with their mortgage payments, we would urge them to get independent advice as soon as possible as we can help them explore options which could prevent them losing their homes."
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