Hello

When you export the amount in currency from invoice lines, odoo export the journal items which are linked to the invoice lines.

If you go check the journal items that were created you will notice that the items linked to the invoice lines are credited (their value is in the credit and their debit is 0)

and the account receivable is debited(its value is in the debit and the credit is 0)

the amount in currency shows the amount in negative if the account is credited( the amount is in the credit) and positive if the account is debited( the amount is in the debit)

for example in the following Invoice 

you can go to the journal items and see the following items

you can also see that when you create a journal entry when you enter the amount in currency it writes the value in the debit, and if you write the amount in negative it will write the amount in the credit.