Trevor Rushforth says he was legally unable to drive his car on April 18 when he stayed in the park at B&M for 12 minutes longer than the permitted two hours.
Mr Rushforth, 72, was having a hypoglycaemic episode, and sat in the car to take his medication after becoming unwell in the store on Church View in the town centre.
It was during the episode that he received a parking fine, and has been ordered to pay £90.
He had an option to pay £45 within two weeks but chose to dispute the fine.
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The Rotherham resident received a letter on Tuesday advising him that the two-week period had expired and he had to pay the full £90.
"As I appealed, I thought everything would have stayed on hold," Mr Rushforth said.
Mr Rushforth said he had no choice but to remain in the car for 45 minutes until his blood sugar rose again.
"Legally, I couldn't go back on the road," he said.
Mr Rushforth spoke to a solicitor, who told him he had a contract with the owner of the carpark,
"It's an unfair contract in terms of this," he said.
Mr Rushforth has suffered from type 2 diabetes for 20 years.
A B&M spokesman said the company was looking into the incident.
“The car park at our Friars Gate store is managed by an independent automatic number plate recognition company, we will be asking them to investigate the appeal as a matter of urgency,” they said.