Air carrier flight crew reported fuel leak during pre-flight inspection. Maintenance was notified and aircraft taken out of service.

Narrative

FO discovered fuel dripping from #2 engine on preflight and brought it to my attention. The FO also told me there was a Maintenance (mx) person at the aircraft and he had let the mx person know of the leak. I entered the mx item in the Logbook. After a few minutes I hadn't seen mx so I called on the radio 129.00 the mx frequency for A gates in DFW. The maintenance guy told us it was our procedure to run the engine and go check it. I replied that we were not maintenance and the item was in the logbook. MX reluctantly agreed to send someone out. After about 20 minutes I still had not heard from anyone so I called on the radio again. I was told very unpleasantly that maintenance was at the aircraft and waiting for me to do the engine run as per my procedures. This is not our procedure. Just after this exchange the maintenance personnel who had been working on the aircraft began walking up the jetbridge. This was the first we had seen them. They came on the plane and told us the fuel leak was from the fuel/oil heat exchanger; was way out of limits and they were taking the airplane out of service. Maintenance tried to direct us to do an engine run for an undiagnosed fuel leak; without maintenance present; then tried to get us to do it again while they had guys inside the engine.Cause: The mx guy on the radio. He's the single issue. Had we not been vigilant in choosing the safe path forward it could have been a very serious situation.