Booking.com They cancel - I foot cost of exchange rate difference

I recently booked accommodation in UK via Booking.com.  I received a confirmation and money was deducted from my account accordingly.  They then contact me to say my dates are now unavailable due to renovations and that I have to cancel booking (free of charge).  I did so but my refund was AUS$43 less than i paid. When I complained about this I was basically told - tough the exchange rate must have changed.  Booking.com was extremely unhelpful. I was not the one wanting to cancel, I received no service yet stil the transaction cost me $43.  I will never use this company again and would warn others against doing so based on my experiences!