The customer explained that he was leaving Malaysia the next day and didn't want the hassle of exchanging the cash.
The man was caught on CCTV fiddling around with the plant.
He did not accept the S$10 out of pride, he said.
They were detected in the sea off Tanah Merah, which is within Singapore's territorial waters.
The Singaporean men, aged 46 and 56, have been detained for 2 days by the Malaysian police.
The 57-year-old suspect will be charged in court for the offence of voluntarily causing grievous hurt.
The restaurant initially believed the man had fairly finished a combined 14kg of curry rice.
Officials confirmed that the duo had unearthed a 15.34-carat diamond.
Clumps of hair were found on the pillow, and the acid had eaten through the bedding.
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