Changing currency code

Hi

Hope you can advise.

A client has a form they want to accept donations from site visitors in either euro , pound sterling or US dollar.

Form can bew viewed here – https://beta.innovista.org/donate/

I am using the gform_currency filter to change this value in the form which seems to translate well , we sbmit between pages and the donation values updates OK , but when we submit to sagepay it only shows the donation value in GBP.

Is there a filter, a value , a form field we can use to tell sagepay we want to handle a Euro or dollar value?

Thanks