How do you know there is a propane tank? Because it was disclsoed, or because you found it in a scan? Are you under contract?
The State of NJ provides grant monies for the removal of oil tanks. I don't believe it applies to propane, but you could check.
More buyers than not will probably want the propane tank removed. You might also consider that since you will someday have to deal with this as a seller.
Good luck with your daughters wedding!
If it's empty, it's not truly a hazard, but empty tanks can often make their way to the surface as soil conditions change, and you don't want that puppy popping to the surface at an inopportune time... say maybe the week before you're going to set up for your daughter's outdoor wedding ceremony on the lawn?