The longer the time until then, the more you will have to pay.
If it is a long time before the next Year of Celebration, the price will be higher, because what is really being sold are the crops that the land can produce.
When property is being sold, the original owner must be given the first chance to buy it.
by paying your owner for the number of years you would still be a slave before the next Year of Celebration.
But any time after that, the price will be worked out according to the number of years before the next Year of Celebration.