I Won't Return the Favor After My Neighbor Watched My Cat

My neighbor Omar watched my cat for 4 days while I was on a trip. I brought him back a nice bottle of whiskey ($60) as a thank-you. Three weeks later, Omar asked me to watch his TWO dogs for a week. I said I could do 2 days but a full week is a lot — dogs need walks, feeding schedules, and one of his dogs has anxiety. Omar said I "owe him one" since he watched my cat. I said watching a cat (leave food out, scoop litter, check in once a day) is not the same as watching two dogs for 7 days. He said a favor is a favor. I offered to help pay for a dog sitter for the remaining days. He said that's "not the point" and I'm being transactional. Am I really obligated to provide equal pet-sitting just because he watched my low-maintenance cat?