What is the Islamic ruling on paying the blood money (Diyya) and giving charity (Sadaqah) in a case where a husband accidentally caused the death of an elderly man in a car accident?

What happened with your husband - which was his accidentally hitting the wrong person, causing their death - is considered accidental killing, which necessitates an expiation upon him. This expiation is: freeing a believing slave; if he cannot find one, then he must fast for two consecutive months. Furthermore, his 'Aqilah (male relatives on the paternal side) owe the legal blood money to the guardians of the deceased, unless they forgive it. If his 'Aqilah refuse to pay it, then it becomes incumbent upon him, unless the guardians of the deceased forgive it. If the court did not rule upon him what is Islamically obligatory upon him, then it is incumbent upon him to fulfill that himself.