Pre-condition on distribution

Shemitta is an instantiation of gift exchange, sheviit is not. 

Sheviit is an instance of the gift giver asking for His gift back. In fact, sheviit is a suspension of the gift, namely, a suspension of the use of the land. 

The use of the land is given to the Children of Israel with a pre-condition: every seven years they desist from using it. It is actually a pre-condition on the distribution of the land among the tribes so that the process of distribution itself is contingent on those who did the distributing to accept the entailment on that distribution. 

Sheviit is thus an extension of the claim to the private property use of the land as it manifests between the national authority who dispersed the claims on the land and the recipients of those claims.