Isca: are you serious that you had such a discussion with a pump jockey?
In my part of the world, which overlaps yours, I tell the guy how much fuel, and he pumps it. Period. He does not get a vote on how much fuel goes in, and I cannot fathom a scenario where you would have to explain yourself to a fuel dealer. Man up!
And if I decide last minute to add 20 tonnes of fuel, it goes on or I don't. Its really that simple.