Add an S to the end of the word and it becomes more then one. Add two S’s and you will end up with one. What is the word?

I’m what you need to understand, but I’m not something you can simply hand. I’m in a joke, a tale, a text, a debate. Remove me, and your words lose sense.

We check on it when buying used things. We fight for it when we negotiate. We can set it. We often accept it without investigating.

I speak the truth by proving it false, and reveal a lie by making it true. What am I?

Should you have it down to a T, then I guess you have no need for me. If an unsolvable riddle comes your way, that's when my help comes into play. It costs you nothing to unlock me. If you know the answer then leave me be!