Eval Value

Try this

in your cs file

public string CheckNumber(object myValue)
{
    if (long.Parse(myValue.ToString()) > 0)
    {
        return "-" + myValue.ToString();
    }

    return myValue.ToString();
}

in your aspx

<%#CheckNumber(Eval("Sold").ToString())%>

try something like the below 

if value >0 ? value * -1 : value;