There have been several cases where escorts were forbidden to leave their role ,which was to protect the convoy, to rescue the crew of a sinking merchant vessel .Convoy tactics/operations varied over the years.
U-boats carried a rather limited number of torpedoes.
Sink large freighter or tanker vs sinking a Flower class corvette, trawler, 20 year old destroyer?
Sometimes there were one or two ships designated as "rescue" ships. Usually small (difficult target) and with sufficient speed to catch up to the convoy.
Since even the fast convoys had ships that ran at 9-13 knots it didn't take a lot of speed to catch up. Rescue ship might be able to cut the corners on the zig zag route also.
U-Boat commanders were usually rated on tonnage sunk.
And that U Boat commanders were usually rated on tonnage sunk was a very big mistake of Doenitz .