I forgot to turn on the Shabbos mode on the refrigerator and my neighbor gave us a new bottle of grape juice that turned out to be spoiled. If we return a new good grape juice bottle wouldn’t that be considered ribis ?
If you didn’t use the bottle, give it back to your neighbor, and politely inform him that the grape juice was spoiled. As far as giving back a good bottle, you are correct, it would be an issue of ribbis, because you are giving him back an item of better quality than what you received, and would be a type of ribbis. The best thing would be to be straight forward about it with your neighbor and straighten the issue out.
Bris Pinchos chap 3 -21.