If you successfully pickpocket the 500 gold reward from Enilroth before he deposits it into the dead drop barrel during the Following a Lead quest, he will still deposit the 500 gold into the barrel as if he still had it, allowing you to essentially double-up on the dead drop gold reward.
Unofficial Oblivion Patch.esp has this source code in Dark17EnilrothScript [SCPT:0006BD10]:
scriptname Dark17EnilrothScript
begin OnPackageDone EnilrothBarrel
Dark17BarrelRef.AddItem Gold001 500
EnilrothRef.RemoveItem Gold001 500
set Dark17Following.Move to 2
end
Maybe it could benefit from checking if Enilroth still has the 500 (or greater) gold in his inventory before also adding the gold to the barrel, to prevent an exploit around receiving the gold reward twice.