The realtor says the propane tank needs to be removed. How serious is this?

Me and my partner have bid on a house. The realtor told us the 1000 gallon propane tank was buried in underground about the time the house was built in 1964. He initially told us the propane tank needed a new valve. Yesterday he calls and tells me the gas (propane) company says the propane tank needs to be taken up and a new one should be rented on the back of this 2 acre yard which sets on the lake. The realtor seems to think it is ok to leave the old propane tank in the ground and run a new line to the new propane tank we need to rent in the back yard. The home owners have offered $300 credit to do this. Does this sound right, or should we not make a deal until the owners have removed the old propane tank sitting in the yard for 47 yrs? I am concerned because the house sits on a lake. Does the propane in the old tank have the potential to cause problems? I will call a propane company today to investigate, but it sounds a lil worrisome to me.