If the advance payment received forms part payment of the supply, GST is chargeable. (information from IRAS)

From IRAS web Site: If you have made a mistake in your past GST return(s), you can correct the error in the current GST F5 return provided that:   The net GST amount in error for all the prescribed accounting periods affected is not more than $1,500; and  The summation of errors in each prescribed […]

  Information from: http://iras.gov.sg/irasHome/page04.aspx?id=1990   You may pay for expenses on behalf of another party (e.g. your subsidiary or customer). Subsequently, you re-bill to recover the expenses from that party.      Whether you need to charge GST on the re-billed amount will depend on whether the recovery of expenses is a reimbursement or disbursement. […]