A319 Captain reported a fuel disagree message during arrival. Flight crew continued to destination and found a fuel leak during post flight walk around.

Narrative

To begin with; we were over fueled the planned amount by 10;000 lbs of fuel. While in cruise flight at XA29 time; we had started the arrival into ZZZ when we got an ECAM - FUEL F. USED/FOB DISAGREE. We ran the QRH and couldn't determine if or where" the fuel leak was due to such a short duration of the flight. All indications appeared typical other than the ECAM message. I was not concerned fuel would be an issue as we had more than plenty for our short flight. I would say though we were definitely tasked saturated during the entire event; arrival and to the short nature of our flight. After landing and at the gate as the passengers were deplaning (we had a relatively light load); my First Officer went to do a walk around and determined we did indeed have a fuel-leak coming from the lower portion of the cowling of the #2 Engine. While the last of the passengers were deplaning; I coordinated with the station to proceed with their fuel spill procedures while also coordinating with Maintenance. Cause: I can't think of anything other than being task saturated as previously stated with starting the arrival while running the QRH.Suggestions: If I were to do anything different; I would have considered notifying ATC to have safety equipment standing by just in case. Not an excuse but as I mentioned before we were quite tasked saturated with the entire event."