Elsewhere Tosafot (Menachot 41b, and Pesachim 113b) express a different view, based on the statement in the Talmud (Menachot 41b) that in “times of anger” G-d punishes for the failure to perform a positive commandment. Due to this risk, the Tosafot in these comments perceive an obligation to acquire a four-cornered garment in order to become obligated in tzitzit. The Mordechai (Pesachim X, 945) disagrees, understanding the risk during “times of anger” to apply to those who go through a process to create an exemption, not to those who are exempt under normal conditions and decline to seek out an obligation. Rabbeinu Yonah (Sha’arei Teshuvah, III, 22) understands a commandment to seek out an obligation only in the case of the mitzvah of tzitzit, which is considered equal to all other mitzvoth (see Menachot 43b).
R. Moshe Feinstein (Resp. Iggerot Moshe O.C. IV, 4) understands there to be an obligation to purchase a four-cornered garment with tzitzit, because of the concern of “times of anger” and because of the minhag Yisrael to do so. Others (such as Resp. Tzitz Eliezer, VIII, 4, and Resp. Yechaveh Da’at, IV, 2) feel there is no risk of punishment in failing to do this but it is nonetheless highly appropriate to do so.
These views and others are analyzed by Rabbi Daniel Rapp (in the journal Beit Yitzchak, XXVIII, pp. 408 – 417) in an extensive essay concerning the question of the obligation to seek out mitzvot that one could otherwise escape obligation from. In the conclusion of this essay, he considers the practice of purchasing land in Israel for the sole purpose of fulfilling the requirements of shmitah. He concludes that is an unproductive practice for at least two reasons: a) a strong view among poskim that even one who owns no land in Israel is already in fulfillment of shmitah; and b)the implication of the Talmud (Nedarim 10a), in reference to the taking of vows, that a mitzvah that involves risks in its obligation should not be pursued. As shmitah would be in that category, it would be unadvisable to seek such obligation.