"I just thought, 'That's not my money, so I can't take it away. I don't know what the hell this money is doing here'."
Mr Amarsinghe was also afraid. "Someone could have put it there and planned to come back for it. I didn't want them to come back for it and find it gone and them come after me."
The incident happened at Australian TV network Channel Nine's Docklands building when Mr Amarsinghe was living in Sydney.
He called his supervisor, and before long police and a plumber collected more than A$100,000 ($109,000) from the bathroom - including almost $1300 pulled from the piping.
The stash sent investigators scrambling, but while a man was charged with dealing with the proceeds of crime, no one was convicted.
No one came forward to claim the cash in August 2011. Now, nearly three years later, Mr Amarsinghe has received a phone call to say most of the money is his. "I was speechless," said Mr Amarsinghe, who is studying IT.
On Tuesday, Melbourne magistrate Michael Smith ruled that the A$81,597 (about $88,200) that spilled onto the tiles in front of the cleaner was his property. The rest will go to the state.
"There's no reason why such honesty shouldn't go unrewarded," he said as Detective Senior Constable Daniel Thorne nodded in agreement.
"All the guys in the office felt the same," said Mr Thorne. "He's a struggling student who straight-up didn't even think of pocketing it."
The court heard Sydney man Emerald Nguyen had been charged. But following a doctor's report in which he claimed he had been involuntarily drug affected, and had no knowledge of the money coming into or leaving his possession, investigators dropped the charges.
Mr Amarsinghe said the money was a blessing. He didn't know how he was going to spend it all, but he would give some to his family, some to disabled people and some to a Buddhist temple in Australia.
Herald-Sun, additional reporting: Vaimoana Tapaleao
