Hello,
Foreign currency valuation:
1. Using FAGL_FC_VAL you can make FX valuation at a certain cut off date, ledgers and methods.
Example: 30.06.2013 for leading Ledger (IFRS) valution in accordance to IFRS.
IFRS you have to valuate with cut off principle.
System is doing the valuation and you can post it to not realized gain/loos accounts.
at 01.07 system can do reversal of valuation.
2.Payments:
Posting for realised gain/loss when performing payment postings is done by system automatically.
3. Summary: You have realised gain/loss and you can do valuation.
These are different points. Only at payment you have realised gain/loss.
4. You can handle valuation 
automatically with FI closing cockpit.
Does this help you?
Otherwise please let me know.
all the best Erwin