Musar zu Dewarim 26:15

We have learned from Rashi on Genesis 18,16 that whenever the expression וישקף, וישקיפו is used, it has a negative connotation, the only exception being the prayer offered by the Jewish farmer in Deut. 26,15 when he pleads for G–d's blessing in recognition of the fact that he has given all his tithes. It seems clear that having given tithes is something which will turn a curse into a blessing. Malachi makes the exact same point; this is why he chose a wording normally associated with negative fall-out. It is a glimpse of the future when the מלאך, the "evil" angel will say Amen, when even the pig will be purified, when the perennial accuser, just as on the Day of Atonement, will be turned into an advocate on behalf of the Jewish people.