How is a remedy different than a marginal social benefit?

If you mean Remedy, it means an exchange for the exact value. while social marginal benefit is the added value returning from satisfying the need.

a perfect example is that you broke my I pod which was worth $800 after depreciation at the second before the accident, kind enough you took it and paid $800, this is a remedy.

you got that I pod now, worthless, so you go to that guy who can fix things, and pay an extra $100 and now you have an I pod. the social benefit is now is worth $700.

You paid the $800 so you do not go to jail, while you paid $100 and got yourself an $800 I pod out of a broken shatter.