Neither Sumner nor Herbert Spencer were social Darwinists - a moniker hung upon them both. What Social Classes Owe to Each Other answers that question with to take care of his or her own self. Minding other people's business is dangerous and wrong. Yet Sumner did think people owed each other compassion and assistance.
There is considerable evidence that the entire concept of "social Darwinism" as we know it today was virtually invented by Richard Hofstadter. It certainly didn't apply to Sumner, who was a great libertarian. The description "social Darwinist" was never made of his views during his lifetime.