@ Sadettin Çeti, we are also using Odoo for our entity in Turkey. How did you solve the situation in Odoo 18? Do you use the Odoo 18 default way with average of buying and selling rate in a productive environment? Or did you change the Odoo code so you only use the buying rate?

I understand from my tax consultant that we have to use the buying rate only and that we may not use an average of buying and selling rate.

I am curious how you solved it for yourself.

The currency rate affects the VAT base.

Legally, ForexBuying  must be used, if there is no mutual agreement.

The average rate is not legal and causes the tax to be calculated incorrectly.

All Rates that Odoo sources from the various currency services are estimates, only placeholders until the real rates are known (during bank reconciliation or sales of assets / settling of liabilities).

Basically any reasonable average is useful as long as the method is consistent, and the mid-market rate =(buy+sel)/2 is commonly used for valuation in financial reporting and accounting. 

The average, or midpoint, balances the buying (bid) and selling (ask) sides to avoid over or undervaluing assets and liabilities that aren't intended to be liquidated immediately at either extreme rate.

Odoo's use of the mid-market rate is in line with how most Accountants reflect a stable and fair market value, especially in situations where exchange rates are volatile.