Hi, is the dog a Bullterrier?
The reason I ask is because they do tend to suffer from dermoid cysts which occur inbetween the toes. One of my BT's suffered from them and they swell up similar to your dog in the picture. In the end they burst and leak fluid.
Might be worth mentioning to your vet as it could save you the fortune of an op if they can treat it with meds.
Best of luck.

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