Creating Payments - Catch22

I have a bit of a catch 22 issue when creating payments. Below are the properties I'm setting.

        oPayment.customer = oCustRef
        oPayment.customFieldList = oCustomFields
        oPayment.tranDate = dtPaymentDate
        oPayment.tranDateSpecified = True
        oPayment.autoApply = True
        oPayment.autoApplySpecified = True
        oPayment.balanceSpecified = False
        oPayment.createdDateSpecified = False
        oPayment.totalSpecified = False
        oPayment.payment = decPaymentAmount
        oPayment.exchangeRate = 1
        oPayment.exchangeRateSpecified = True
        oPeriod.typeSpecified = False
        oPeriod.internalId = 1
        oPayment.postingPeriod = oPeriod
        oPayment.externalId = strExternalID

When I use these properties I get the error 'please supply a payment amount.' Given that I am supplyng the amount I figure it must be talking about the 'total' property. If I change the 'totalspecified' to 'true' and set the 'total' value I get 'You do not have permissions to set the total due'. What's funny is that I'm not trying to set the 'total due', I'm trying to set the 'total' as per the original error. Either I'm missing something obvious or the error message is not correct. I hope it is just that I'm missing something obvious.