Sforno on Leviticus 25:15/ספורנו על ויקרא כ״ה:ט״ו

1במספר שנים תקנה, you are to pay a price corresponding to the number of years that you will own the land. Our sages state that when it comes to matters which are subject to verification by weight, by measure, or number, even if the overcharge amounted to less than 1/6th, the usual leeway when buying things in “bundles,” etc, is refundable. (Baba Metzia 56)

2אחר היובל תקנה, there is no point in purchasing a field during the actual Jubilee year, as a situation might arise in which the returning owners cannot retain possession of it for any meaningful amount of time at all, depriving them of the feeling that they had benefited from the Yovel institution.

3שני תבואות, years during which a harvest is produced, not years of drought, etc. The conditions are parallel to a farmer who enters into a tenancy agreement working the field for the owner and giving him a portion of the harvest.