First step to synchronize your prices correctly you have to choose proper currency in 'Manage'/'Online Portals'/'Room map'
It has to be the same currency as the one, which is set up in your back office!!!
What can you do if currency in your back office is different than your default currency?
Options:
1) Contact with Booking.com management and ask them for changing yours back office currency.
2) Go to 'Manage'/'Rooms and Rates' and and set prices for currency used in back office - then you will have fixed prices in currency used in back office and costs in other currencies will depend only on current exchange rate.
Don't forget to actualize the exchange rate everyday !!!
3) Go to 'Manage'/'Rooms and Rates' and and set prices for default currency - then you will have fixed prices in your default currency and prices on Booking.com will depend only on current exchange rate. Don't forget to actualize the exchange rate everyday !!! Unless you are risking getting bookings with wrong prices !