(Chapter 7 case; debtor's wife died and debtor entered into family settlement agreement with remaining heirs which left cattle ranch to heirs, subject to debtor retaining right to use profit from land and cattle for personal living expenses; after debtor failed to pay creditor for hay, creditor awarded judgment; debtor filed bankruptcy petition and trustee claimed that probate estate and others owed debtor funds for cattle-related expenses and for land rent; bankruptcy court rejected trustee's claim for unjust enrichment under 11 U.S.C. Sec. 542(a) because such claim beyond statute's scope; 11 U.S.C. 542(a) also required that probate estate have present possession, custody or control or property that trustee sought recovery for, and possible possession at end of debtor's life estate did not satisfy statute).