Beware lest there be a base thought in your heart, saying, "The seventh year, the year of release, is at hand," and your eye be evil against your needy brother and you give him nothing, and he cry out to Hashem against you, and it be sin in you.
English: "And it be sin in you" — Rabbi Elazar said: Come, let us be grateful to the deceivers; were it not for them we would be sinning every day, as it is said "and he cry out to Hashem against you and it be sin in you."
On the daf / in context: Because many who solicit charity are not genuinely needy, the average person is spared constant liability; the verse is understood to apply only when the need is certain.
Type of Learning
Derash on the verse (Kesubos) — gratitude to the unworthy poor for limiting liability.
What Is Learned
Were it not for the presence of unworthy solicitors, every refusal of a request would constitute the sin described in the verse.