Export sales fob or invoice value

My client From 2011 - 2012 till mid of 2015-2016 whatever export sales have shown in fob value as sales and same have credited in customers account duely receivable 

but thing is invoice value should be have credited instead of fob value in customer account due to what ever extra we need have received is profit due to currency fluctuation ++

So I need your help how to show this unaccounted facts in accounts for 14-15 and 15-16.