The below information is from IRAS web site: http://iras.gov.sg/irasHome/page04.aspx?id=676
For such purchases with GST, your supplier has to indicate the GST payable on the tax invoice in Singapore dollars at the approved rate of exchange determined by him. You should claim input tax on such purchases based on the Singapore dollar amounts shown in the tax invoices. This requirement applies even if you have recorded the purchases at different exchange rate in your books.
For your imports, you should claim input tax based on the Singapore dollar amounts shown in the import permits issued by Singapore Customs.
The above information from IRAS web site is very helpful for those who are importing goods into Singapore in foreign currency.