There is a reason why hiring halls acted as bonding agencies -Tracking debts and payments as per contact. A mercenary unit isn't an informal club, it's a business that's registered to operate within a realm, with all the rights and obligations that entails, such as paying taxes and being subject to regulations and taxation. Hiring halls and government of register also provide mediation for disputes (court systems, appeals boards, etc.,) so if a unit is eliminated, the employers are still obligated, by letter of contract, to pay for services rendered. A "failed mission" does not overrule payment for travel, expenses and material losses, which is why a bonding agent holds a portion of the payment in escrow, should the employer renege in legal payment.
As a business, a mercenary unit may have assets, property, shareholders, lenders and other obligations far from the battlefield. If a merc unit is destroyed and the employer reneges, the host government steps in -They own a portion of the payment in taxes and if there are next of kin, they take inheritance taxes, as well.