Clive Everall, 54, from Shrewsbury, had been teaching English at a school in the country.
He said the Thai authorities had accepted the incident was accidental, but when he had attempted to return to the UK he was arrested, and has since been asked to pay compensation to the man’s family.
Mr Everall had been volunteering in the country for four months when he hit the 77-year-old on January 29.
He had been teaching English at Uttaradit High School on the morning of the incident, and says he was fully sober and driving safely but “could not avoid” the man.
Mr Everall says he was told by police that he had not been at fault.
He said: “The police said, ‘Don’t worry, it was an accident’, and I was backed by the agency I was working for.
“I attended the funeral, I held the man’s hand in his coffin and hugged all his family, and went to the cremation.
“I have done everything I can to be respectful.
“Then last Monday I had a message asking me to go to the police station.”
At the station, Mr Everall says he was told he would have to pay compensation to the family, despite no criminal charges being brought.
Mr Everall told the police he only had 20,000 baht – around £500 – but said the relatives were seeking up to 10 times this amount.
“The police told me to go back home raise funds, come back and renegotiate with the family,” said Mr Everall. “I signed a paper then I was told I was free to go.”
He said he travelled to Chiang Mai Airport to board a flight back to the UK, not knowing a warrant had been issued for his arrest.
He was detained by immigration officials and taken back to Uttaradit.
Mr Everall said he was left not knowing what was going on. He said: “The next day I made a statement with a Thai lawyer and an interpreter who wasn’t very good. I refused to sign all the police paperwork because it was all in Thai.
“I went to the court house and the judge said I would be given a tag on my leg.”
Mr Everall now faces uncertainty over what will happen at his next court appearance on Wednesday, but fears he will be jailed if he cannot come up with the money.
He is now pleading with the Thai authorities to be allowed home, where he can return to work in order to pay the compensation.
“I just want to come home,” he said.
“My plan is to negotiate a payment plan with the family and be allowed to leave the country. I have got work when I come back. I want to do what’s right but I can’t do anything for them if I am locked up.
“I am trying to remain strong and keep my head high. I am a caring man and I had come here to do something good.”
Mr Everall has set up a fundraising page, and has so far raised £1,800 towards the compensation and his legal fees.
A spokesman for the Foreign and Commonwealth Office said: “We are providing consular assistance to a British man who was arrested in Thailand, and are in touch with his family.”