1)
What is "Sefas Yeser"?
1.
Rashi: It is words of pride.
2.
Malbim: If one does not do all that he promised, he spoke Sefas Yeser.
2)
Why does it say "Af Ki l'Nadiv"?
1.
Rashi: All the more so Sefas Yeser is not proper for a Nadiv.
2.
Malbim: Naval and Nadiv are opposites. Even though a Naval does not normally promise gifts and Chesed, if he does promise, not only must he fulfill it, to avoid Sheker. He must guard all that he promised, to avoid Sefas Yeser, which is reproachful. All the more so a Nadiv must [do like he promised, to] avoid Sefas Sheker, for three reasons. (a) He is a Nadiv. (b) He must fulfill his promise. (c) He must not speak Sheker.
