What is the Sharia and statutory basis for obligating the defaulting party to compensate for the legal fees incurred by the right-holder?

This is based on the fact that the obligor, by delaying payment, has compelled the right-holder to appoint an attorney to claim his right, thereby necessitating compensation for the resulting damage. This relies on the Sharia maxim: 'Harm must be redressed' (الضرر يزال), and the statement of Sheikh Al-Islam Ibn Taymiyyah: 'If the party obligated by the right is capable of fulfilling it, but delays his counterpart in the right until he compels him to file a grievance, then whatever he spent or incurred due to that is upon the unjust wrongdoer, provided that the expenditure was customary and reasonable.'