I think they will bill in RMB if it is CNY transactions.
http://www.icbc.com.sg/personal/Introduction of Payment Methods.pdf
CASH PAYMENT
Cash payments should be made over counter at 6 Raffles Quay #01-01 with your identification and credit card no. You can choose Repayment of credit card RMB account debts using RMB, Repayment of credit card SGD account debts using SGD, or Repayment of credit card RMB account debts using SGD by purchasing RMB. The exchange rate will take the market exchange rate on the payment day.
To learn the exchange rate, you could call the 24-hour hotline (65-63695588).
http://www.icbc.com.sg/personal/Introduction of Dual Currency Credit Card.pdf
Features
* Tailored and unique design featuring dual currency accounts
Apart from the basic functions of overdraft, spending and transfer, the product also features dual currency accounts in RMB and SGD for cardholders’ convenience when travelling to and from China. Cardholders can spend money in RMB on the Chinese Mainland and SGD in Singapore and the rest of the world. As a result, cardholders can easily make settlements in RMB for the spending in Mainland China without worrying about the exchange fees.
* Two accounts with one shared credit limit for carefree spending
There is a shared credit limit for the two accounts, which means cardholders can spend as much as they like within that shared limit, either in RMB or SGD.