Negative commandment · Lo Ta’aseh #231 of 613
To not refrain from lending before the sabbatical year
שלא יימנע מלהלוות לעני מפני השמיטה
The verse · Devarim 15:9
הִשָּׁמֶר לְךָ פֶּן־יִהְיֶה דָבָר עִם־לְבָבְךָ בְלִיַּעַל לֵאמֹר קָרְבָה שְׁנַת־הַשֶּׁבַע שְׁנַת הַשְּׁמִטָּה וְרָעָה עֵינְךָ בְּאָחִיךָ הָאֶבְיוֹן וְלֹא תִתֵּן לוֹ וְקָרָא עָלֶיךָ אֶל־ה׳ וְהָיָה בְךָ חֵטְא
“Beware that there be not an unworthy thought in thy heart, saying, The seventh year, the year of release, is at hand; and thy eye be evil against thy poor brother, and thou give him nothing; and he cry to the Lord against thee; for it shall be reckoned to you as sin.”
The Rambam’s words
Not to withhold lending money to a poor person because of the advent of the shemitah year, as [Deuteronomy 15:9] states "Be careful, lest an idea...[occur to you....]" This is an accepted general principle: Whenever [the Torah] uses the expressions "Be careful," "Lest," or "Do not," a negative commandment is involved.
שלא יימנע מלהלוות לעני מפני השמיטה, שנאמר "הישמר לך פן יהיה דבר" (דברים טו,ט). .זה הכלל--כל מקום שנאמר הישמר או פן או אל, אינו אלא מצות לא תעשה.
Why · Sefer HaChinuch #480
To not refrain from lending before the sabbatical year
“It is from the roots of the commandment [that it is] to strengthen and to fix the trait of generosity in our hearts and to completely distance the trait of stinginess.”
Who & when
“And the prohibition is practiced by males and females in any place and at all times.”
Where in the Mishneh Torah
Hilchot Sabbatical Year and the Jubilee הלכות שמיטה ויובל