Well, today $40 USD = £25 GBP according to one source I just checked.
Or, using the exchange rates you posted earlier, your store would be reporting £26.32 if the price was $40 USD.

So, I have no idea what your client is referring to. Are they talking about product prices changing incorrectly when the different currency is selected? Are they talking about order history showing wrong amounts? Are they complaing that after they collect payment and pay service charges for the transaction that they're not left with all the money they wanted? And so on.

You need to work with your client to figure out what exactly they're referring to, what exactly they think is wrong, why they think that's wrong, what they think it should be and why, work through some scenarios with them, and come back with some specifics.

You also need to indicate more information about versions, addons, customizations, etc. When you click the "Reply" button below, there are several questions in the "Posting Tips" section which you should provide answers to when you post again.