Emirates Conditions of Carriage states in 4.1 "The fare for your Ticket has been calculated according to our Tariff as it applied
on the date
payment was made for your Ticket."
So, payment date is the key.
They also say in 4.2.2 "Taxes, fees and charges imposed on air travel are outside our control and are constantly
changing and can be imposed or changed after the date your Ticket has been purchased. If a
tax, fee or charge is imposed or increased after your Ticket has been purchased, you must pay
to us any such tax, fee or charge, or any such increase before carriage."
Which basically means that they can come back and ask you for more money if fuel costs increase for instance (aka Fuel Surcharge).