It depends on what kind of foie gras you have.

If you see "cuit" on the container your foie gras is cooked.
If you see "mi-cuit" on the container your foie gras is semi-cooked.
If you see "frais" on the container your foie gras is fresh or raw.

It has been my experience that a great deal of the foie gras served in France (especially in restaurants) is the raw variety, while generally in the US it is cooked.

Since yours is in a jar,it was probably cooked to increase its shelf-life. I imagine since it is probably cooked you should be safe at customs. Though it may depend on the individual agent.