I would insist that they remove the fill line, for the reason mikes pointed out.
Horror story: friends of mine bought a house. One of the big selling points was the brand-new gas furnace that had replaced an oil furnace. The house was inspected by a professional as a condition of sale.
They came home one day to the stench of diesel fuel. No one had told the fuel oil company to stop their regular delivery, which they had duly deposited, in accordance with the never-cancelled contract with the former owner, in the still-in-place filler. But the tank had been removed, and 1,200 gallons of fuel oil covered the floor of their basement to several inches.
Last I heard -- three years later -- the only ones who had benefitted were lawyers. My friends were heartbroken and nearly bankrupt. The former owners, the home inspector, the fuel oil company, and the new owners were all pointing fingers at each other. The US EPA was involved. The damage was into six figures. The entire concrete basement floor and the bottom two feet of the house's footing had to be removed and hauled to a certified toxic waste site -- not to mention possessions they had in the basement. They had to live in a motel for about six weeks.
About this time, the housing bubble burst, the bank got wind of the problem, and called in their loan. They were told the house could never be sold again, with state disclosure laws and such. The lawyer got the mortgage continued, but of course at a pound of flesh.
So NOW is the time to head off a potential disaster!