The proposed workaround is:

 Following extensive investigation, the behavior is intended as design in the code below:

TransactionReversal_Vend.reversal()

if (isAdvanceReversal)

{

 reversalDate = DateTimeUtil::getSystemDate(DateTimeUtil::getUserPreferredTimeZone());

}

 In addition, following investigation, we can also reverse prepayment invoice by click on remove prepayment functionality on the purchase order form.

The problem: Reversal date is the system date and not the invoice date or the session date. It would be better to follow the function of reversal of vendor transactions: to chose the reversal date. Otherwise I would have problems in my year end reports (balace sheet and profit and loss) if prepayment invoice posting date is different from reversal transation date!