Engines and parts (and possibly maintenance contracts and...

Engines and parts (and possibly maintenance contracts and leasing arrangements) are, as I've always understood it, denominated in US Dollars. That protocol, even if informal, gives everyone in the MRO industry a common basis from which to deal.
As Prime's assets and trading results are naturally in USD, exchange rate adjustments are automatically brought to account.
OTOH, inventory FX adjustments across the whole group are at the discretion of the Board. Whether that's the case here and the profit results are being massaged against the windfall property profit, I'm not sure. But it doesn't matter. I don't think the FX is due to any performance issues or 'overpaying' at Prime.