Riu have a best price guarantee. I booked 7 nights at one of their hotels in Agadir and then found it cheaper on booking.com. The booking.com booking had a free cancellation period of 5 days whereas the Riu booking was 7 days.
They would not honour the price difference stating as below.

In this case, we have reviewed your case with our internal department and due to the terms and conditions of the third party websites not matching our internal terms and conditions, unfortunately the Best Price policy does not apply to your reservation.

The cancellation policies are different.

They should have honoured the price and cancellation period.