HI Boris,
If i understand your issue correctly - you expect to post only the exchange difference from previous F.05 run on Dec31.
In Classic GL F.05 posting system by automatically posts the summarized exchange differences every period end when you execute the program and reverses on next day of program run.
if the requirement is to post the only difference from previous exchange valuation run, then the new GL FAGL_FC_VAL program Delta posting feature will provide the needed solution.
Cheers,