Quoting from
https://vouchers.cdc.gov.sg/residents/faq/
Do I get my change back if the purchase (e.g. $8) is less than the amount of digital vouchers that I want to spend (e.g. $10)?
Similar to other vouchers like supermarket vouchers, there will be no change provided if the purchase amount is less than the vouchers used.
So, not supposed to get back change. I don't think it is illegal in the sense of getting fined/jailed for vendors to give change back.
More like it's discouraged to prevent possible abuse where people use higher value vouchers to pay for something much cheaper, in order to turn it into cash.
Will also create immediate problems for the vendor. Eg if 1,000 customers a day buy 80 cents kopi-o with their $10 vouchers to get back $9.20 in cash, the vendor has to have $9,200 in cash to give out daily.
IRAS will also see that vendor made at least $10,000 sales a day via vouchers. Later file tax returns the vendor also jialat...