What is the expiation for quarreling with a customer whose whereabouts I do not know, especially given that the deeds of those who dispute are not raised [to God]?

Do not abandon a Muslim unless there is persistence in animosity, enmity, and the abandonment of greetings, according to the Hadith: "It is not lawful for a Muslim to abandon his brother for more than three nights, where they meet and one turns away and the other turns away. The better of them is the one who initiates the greeting of peace." Greet him so that what is in his heart may dissipate and you may reconcile.