Social surplus is the amount of welfare (value or utility) that a society has gained from the present consumption of all goods and services produced or bought. It is not something that can be used like money or resources. Example: national security is a type of welfare because it allows people to feel secure about their life and property. So if a country has been investing in her military, the part of her social surplus that military investment has produced is a sense of being secure and confident about the present and the future. In this context, "surplus" means "the value placed on a feeling of being secure, minus the cost of investment that has been made to bring about that feeling." It is as if the society had determined (e.g. through a political process) that a feeling of being secure is worth 10 million hours of "productive work." And, the cost of providing this secure feeling takes only 2 million hours of work. The social surplus is then 10 - 2 = 8 million hours of work that the society was ready to do but didn't have to.
But you may be using the term "surplus" in a different context. If so please clarify.