project cost vs revenue and invoice amounts differs in work based billing

Hi,

We use work based billing and revenue methods.

We understand that, the GDR or GDI programs takes the exchange rates as per current month to calculate amounts for non-project functional currency transactions.

project currency, functional currency = USD

expense report currency: SGD, project expense report recorded in oct-15 in SGD.

customer and contacts form: USD

project revenue and invoice generated for above expenditure in dec-15, as per oct-15 exchage rates of SGD to USD.

the revenue amount is slightly different from costed amount as per dec-15 exchage rates.

customer would like to bill same amount what we spent in oct-15 in the expense report.