As confirmed by Microsoft Support the option for Fixed FCY for Price (and Cost) only works when a Project is setup in a Foreign Currency and the Project is to be billed in another Foreign Currency. If the Project is setup in the Local Currency then when amounts are billed, they will always use the Exchange rate as per the Posting Date selected during the Create Sales Invoice process. We would like the ability to define a Foreign Currency against the Planning Line and to then fix the amounts to be billed in that Currency. The same to be applied for Costs in fixed Foreign Currencies.