Commentary for Deuteronomy 15:8

פתח תפתח [BUT] THOU SHALT SURELY OPEN [THINE HAND] — even many times (Sifrei Devarim 116:13).

כי פתח תפתח, "for you shall surely open, etc." What made Moses think that such a reason would open the hearts of insensitive people? Perhaps Moses referred to something we read in Proverbs 11,24 that "one man gives generously and winds up with more;" being charitable does not diminish one's wealth but increases it. When the Israelite "opens his hand," G'd "opens the gates of heavenly bounty for him." This is why Moses said כי פתח "in order that G'd will open His hidden treasures, etc." He continues with והעבט תעביטנו, "you shall lend him whatever he lacks, etc." The Torah means that when you open your hand for the poor all you are doing is giving him a loan seeing that G'd will repay you with interest. This is certainly sufficient reason for Moses telling you to open your hand again and again.

כי פתוח תפתח את ידך, but