Reference for Sheviit 6:4

מֵאֵימָתַי מֻתָּר אָדָם לִקַּח יָרָק בְּמוֹצָאֵי שְׁבִיעִית, מִשֶּׁיַּעֲשֶׂה כַיּוֹצֵא בוֹ. עָשָׁה הַבַּכִּיר, הֻתַּר הָאָפִיל. רַבִּי הִתִּיר לִקַּח יָרָק בְּמוֹצָאֵי שְׁבִיעִית מִיָּד:

When is a person allowed to buy greens during the post-Sabbatical year? Whenever that species [of greenery] is producing anew. Once the [produce in a place that produces an] early crop is ripe, the [produce in a place that produces a] later crop is permitted. Rabbi permitted the buying of greens immediately upon [the beginning of] the post-Sabbatical year.

Jastrow