How can I safely rehome or surrender my dog for free if I can no longer care for him?

Due to an unexpected cross-country move for a family emergency, I’m heartbroken that I can’t take my 3-year-old Boxer mix with me. I want to make sure he goes to a safe, loving home without having to pay high shelter surrender fees. Has anyone used reputable rehoming sites or no-kill rescues that don’t charge a fee?