Solution : https://service.sap.com/sap/support/notes/1099913 (SAP Service marketplace login required)
Key words : 
exchange rate difference line, exchange rate difference, terms f5 f80, foreign currency invoices, foreign currency postings, local currency amount, local currency tax, foreign currency, local currency, tax manually
Related Notes : 
       
| 1222923 | F5 580 Balance in 2nd/3rd local curr w/ manual tax in LC | 
| 1171092 | F5 060 w/ direct tax in FB60/FB70 Argentina/Peru | 
| 1145030 | F5 580 in second/third local currency in batch input | 
| 1112432 | F5 580: Balance in second/third local currency, manual tax | 
| 994329 | F5 580 Incorrect tax amount in second/third local currency | 
| 645024 | Incorrect tax amount in second local currency |