En la divisa base
...las tarifas confirmadas se garantizan en la divisa base del hotel.
Not quite sure what "en la divisa base" del hotel means there. Can anyone help, please?
base currency - it's an accounting term.
They may have quoted you a rate in your currency ($200/night, for example), but they're not going to be held responsible for exhcange rate fluctuations, so the rates are only guaranteed in base currency.