Hi,
Normally transactions will be posted with the exchange rate the one which has been maintaine for exchange rate type M.
For the valuation purpose you need to maintain a separate exchange rate type, and for this type closing rate will be maintained. This exchange rate type has to be assigned in the valuation method.
This valuation method again will be assigned to valuation area. In that way month end balances will be valuated with a different rate.
Rgds
MBN