The Scottish Labour party's leader has admitted that she wrote personally to the businessman who made an illegal donation to her leadership campaign.
Wendy Alexander had earlier apologised for the row over the donation from Paul Green, which broke rules outlawing donations from people based offshore.
The businessman revealed Ms Alexander had written to thank him for the cheque - one of two he gifted to Labour.
The SNP said the Labour leader's position was becoming "untenable".
However, her team said she had done nothing wrong and would be "toughing it out".
The Electoral Commission has said it will examine all donations made to Ms Alexander during her leadership the campaign.
Mr Green, who is based in the Channel Islands, gave £950 to that campaign at the request of Glasgow MSP Charlie Gordon.
Mr Green revealed on Friday that he had earlier given a similar amount to the Glasgow South branch of the party.
In a statement, the 65-year-old said no questions had been raised when he made the donation in April.
Mr Green said he had received a personal letter from Ms Alexander thanking him for his later contribution.
"I am very angry after innocently becoming embroiled in a national controversy surrounding donations to the Scottish Labour Party," he said.
"However, I have been completely exonerated. My donation to the Wendy Alexander campaign was open and above board.
"When asked if I could donate, I was told there was no problem."
He said the row had caused "unnecessary" damage to the party in Scotland.
"I cannot understand why they continued to maintain that the donation had come through a UK company when I had a letter from Wendy Alexander thanking me personally," he added.
In a short statement, a spokeswoman for Ms Alexander said: "As with other donors, Wendy Alexander wrote a thank you letter to Paul Green.
"As said previously, we will continue to co-operate fully with the Electoral Commission."
The spokeswoman also confirmed that Ms Alexander had "regulated donee" status.
This means she is liable to check the admissibility of all donations above £200 - such as the gift from Mr Green.
Tory deputy leader Murdo Fraser asked: "When Wendy Alexander signed her letter to Mr Green, did she not think to query the Jersey address?
"Is it credible to suggest that the Scottish Labour leader, with a long track record of political experience, would have been totally unaware of the law on political donations?"
However, Ms Alexander's team said she had been as much in the dark as others because they believed it had been a corporate donation.
In her statement issued earlier, Ms Alexander told reporters: "I deeply regret that this situation has occurred.
"I am very sorry for it. I realise that you will have many questions but it's a matter for the Electoral Commission.
"They are looking in to it and it really is inappropriate for me to comment on any specifics at this time."
Mr Gordon confirmed on Friday that he had asked Mr Green to make the donation in April.
He said he had written to the chairman of the Glasgow South party after he realised the "invalidity" of the latest donation.
Mr Gordon has apologised to the party chairman and to Mr Green, and stressed that he had acted in good faith.
"I am not aware of any other donations made by Mr Green to any part of the Labour Party or to any Labour elected member," he added.
Labour said it was not told about Mr Green's earlier donation to the Glasgow South constituency.
The party said the donation should have been referred to its compliance unit - and would have been rejected if that had happened.
The SNP said it expected the Electoral Commission to refer matters to the police for investigation.
MSP Roseanna Cunningham said the responsibility lay with Ms Alexander.
"One way or another, there must be a police investigation," she said.
"Wendy Alexander's position is becoming increasingly untenable - by the day and by the hour."