TT #301 · Parashas Reeh · Sixth Aliyah · Passuk 15:9
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.
Torah Temimah points to Sifrei.
English: "And he cry out against you..." — One might have thought it is a mitzvah for him to cry out; therefore the verse says elsewhere "and he shall not cry out against you." One might have thought that if he does not cry out there is no sin in you; therefore the verse says "and it be sin in you"—in any case. If so, why does it say "and he cry out against you"? I hasten to exact payment through the one who cries out.
Derash on the verse (Sifrei) — the cry accelerates punishment but is not required for liability.
Withholding charity is a sin whether or not the poor person cries out; when he does cry out, divine retribution is hastened.