IME, online DL purchases are always charged in the currency of the originating country. (US Cdn HK UK)
E-credits are converted to local currency and then applied to the ticket. I was never fortunate enough to use an e-credit with residual value so I do not know how the unused portion in different currencies is treated. From my international business experience it is sometimes you win and sometimes you lose in these currency exchange situations. Most multi-nationals are understanding and will make good your fx losses.