acabalar

To complete or round out something that's short of a full amount — making up the missing piece of a sum, a set, or a story. A literary, somewhat old-fashioned verb; completar is what most speakers would say instead.

• Le faltaban veinte pesos y un vecino se los acabaló. — He was twenty pesos short and a neighbor made up the difference.
• Con ese último libro acabaló la colección completa. — With that last book he completed the full collection.
• El relato acabala los sucesos que el primer testigo dejó incompletos. — The account fills in the events the first witness left incomplete.

This verb is rare enough today that completar or 'completar lo que falta' will be understood far more reliably than acabalar.

Often confused with completar.