Sforno on Deuteronomy 16:17/ספורנו על דברים ט״ז:י״ז

1איש כמתנת ידו, not that he has to give away all that he owns so that as a result he will become a charge on public charity; an interpretation offered by foolish gentiles. Our sages have long ago postulated that even the most generously inclined person should not give away more than one fifth of his wealth. (Ketuvot 50)