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What's a good non-cash compensation/show of appreciation to a neighbor for cat-sitting?
If he's the type of guy that doesn't expect repayment and did it out of the kindness in his heart, you can reciprocate by doing the same. Let him know you owe him one, and make sure sure it's received that you're serious and not just something "you say".
When your time comes to do something for him, do you expect payment? Or for him to grovel?