As you all must already know, Batam island is part of Indonesia, governed by Indonesian people. Regarding the currency, Batam has a very unique situation. For some reason, Singapore Dollar is more popular than Indonesian Rupiah. Many people do business in Singapore dollar, and not Indonesian Rupiah. If you happens to find a townhouse to buy or rent, or if you have any machine in need, the seller will usually give you price in Singapore dollar.
Many people find it weird, but they do it for their own reason. First reason, Singapore dollar is more stable than Indonesian rupiah. If you do business in dollar, the price will be more stable. Second reason, some people said that Singapore dollar is simpler than Indonesian rupiah. For example, for one cup of coffee, you can charge a customer S$1, or Rp7.500. People rather to write only one number to four numbers. Third reason, there are many expatriate at Batam island, and most of them are Singaporean. Many local businessman often do business with Singaporean, and they find it convenient to do the transaction in their currency.
This is a very unique situation for Batam, but also an opportunity to grab Singaporean customer, because they will find it convenient, and feel at home. Local and small business owner will still use Rupiah, but for bigger business deal, they use Dollar more often.