When an invoice issued in foreign currency is paid later at a different rate, a difference in lira appears. Under article 24/c of the Turkish VAT Law this difference has been part of the VAT base since 2019 and must be documented with an invoice. If the rate went up, the seller invoices the difference to the buyer; if it went down, the buyer invoices the seller.
Revaluing open foreign currency balances at the current rate at month or year end is a different matter. A difference that comes from a revaluation under article 280 of the Tax Procedure Law carries no VAT and needs no invoice; it is recorded with a voucher.
In TEON both differences come to the same screen, the Exchange Difference Voucher. It matches the customer’s foreign currency movements first in, first out and shows on each line whether the difference comes from a collection or from an open balance. For a collection difference, opening the invoice brings up a sales invoice if the difference is in your favour and a purchase invoice if it favours the other side, already filled in; the VAT is separated from the difference and you save it yourself. You can also close that line with a voucher instead.