Related for Chullin 270:24

A Jew and a Gentile that bought a field in Syria, behold they are like untithed produce and like tithes that are mixed together, the words of Rebbi. Rabbi Shimon ben Gamaliel says, the portion [belonging to] the Jew are liable in tithes, and the portion [belonging to] the Gentile are exempt from tithes. A Jew that bought a field in Syria, even though he went back and sold it to a Gentile, [the field] is liable in tithes and in shevi'it (the laws of the sabbatical year), because it had previously been liable. But [in the case of a field owned by a Gentile but worked on by Jewish] sharecroppers or tenant farmers or the families [of Jewish sharecroppers], or [in the case where] a Gentile mortgaged his field to a Jew, even though he acted based on [Jewish] law, [the field] is exempt from tithes and from shevi'it [because ownership is retained by a Gentile, see Gitt. 43b:11].