Mr Aslam, a marine worker, was hoping to be promoted at his worksite after attending a safety supervisor course in May.
A promotion means that the 32-year-old Bangladeshi's basic monthly pay would be doubled to $1,200, and he need not slog under the hot sun.
He did not mind paying $500 for the four-day course, which usually costs about $250, as he was promised that he would find it easier to pass and he needed to go for only one day of the course.
"I thought that it was the way things are, like how things are in my country... I pay more and I can do less," he said.
But he ended up having to pay another $250 for retraining after the Manpower Ministry (MOM) revoked the accreditation of the company that conducted his course, Regent Global. "We didn't learn anything. (The company) gave us books, and we spent one Saturday night reading and got our paper (certificates)," he said.
Responding to queries earlier this month, MOM said that since 2010, it had received an average of 30 complaints a year against trainers it accredited.
In June, MOM said it had cancelled the accreditation of five training providers, including Regent Global, between December last year and May, affecting about 500 workers. Their accreditation was cancelled for the "failure to conduct courses according to stipulated course durations, failure to ensure trainees' language proficiency, and failure to uphold the integrity of examination procedures", said a spokesman.
In the past two years, nine training providers lost their accreditation, making a total of 11 since May 2010. MOM did not say how many letters it had sent to inform companies that their workers needed to be retrained. But safety managers speaking to The Sunday Times estimated that at least 1,000 workers have been affected since the beginning of last year.
Ten marine and construction workers from India and Bangladesh interviewed said they had gone for courses where the trainers allowed them to copy answers off a screen and helped them to change answers. They also did not attend the whole course before they were certified.
It is difficult to gather proof of the malpractices of these training centres because workers could not take their phones into the centres, workers said. Guards are also stationed outside to alert the trainers if MOM officers arrive to conduct checks.
On two occasions when The Sunday Times visited the errant centres in July, workers walked out with course certificates, even though they did not take tests for the courses.
Workers said that the training companies had approached them through an agent, who is usually from their home country.
"In our country, we pay more (for agents). I didn't think I will be cheated," said Bangladeshi construction worker Islam, 28. "Now, he is uncontactable."
Mr Aslam, who was recommended an agent by a friend, attended the course in May, but the training centre in Upper Paya Lebar Road had its accreditation revoked in March.
According to the Singapore Institute of Safety Officers (SISO), the affected courses are usually those for working at heights and a mandatory training course for supervisors that cost between $250 and $300.
SISO president Seet Choh San said: "Training and assessment malpractices for supervisory courses put innocent lives at risk as these supervisors would not have had the requisite knowledge to prevent injury or ill health to workers."