Malaysia-registered cars can pay a fixed rate of $5 daily to use ERP-priced roads during ERP operating hours, regardless of the number of times the vehicle passes through the ERP gantries within the same day. The fixed ERP fee will be deducted through your Autopass Card when you exit Singapore.
Alternatively, You can also choose to rent a temporary In-Vehicle Unit (IU) at the Tuas or Woodlands Checkpoint to pay your ERP charges. The IU rental costs $5.15 a day for a minimum rental duration of 2 days, for up to 2 weeks. You will also need to pay a refundable deposit of $124.65.
Info from: https://www.onemotoring.com.sg/content/onemotoring/home/driving/entering_and_exiting_singapore/cars-and-motorcycles-registered-in-malaysia.html