Now I think, I understood your question. Thanks.
Oracle ERP (like any other enterprise scale ERP) is designed to be used by all kind of companies.
If your client follows such requirement than you can advice the client to enter settlement dates for prepayments; and keep the settlements date in the next period so that prepayment could not be applied in the current period.
But yes you are right. Oracle will let you apply prepayment even in the current period if you keep settlement date as system date.