Vayeira - Looking Back

(15) As dawn broke, the angels urged Lot on, saying, “Up, take your wife and your two remaining daughters, lest you be swept away because of the iniquity of the city.” (16) Still he delayed. So the men seized his hand, and the hands of his wife and his two daughters—in God's mercy on him—and brought him out and left him outside the city. (17) When they had brought them outside, one said, “Flee for your life! Do not look behind you, nor stop anywhere in the Plain; flee to the hills, lest you be swept away.” (18) But Lot said to them, “Oh no, my lord! (19) You have been so gracious to your servant, and have already shown me so much kindness in order to save my life; but I cannot flee to the hills, lest the disaster overtake me and I die. (20) Look, that town there is near enough to flee to; it is such a little place! Let me flee there—it is such a little place—and let my life be saved.” (21) He replied, “Very well, I will grant you this favour too, and I will not annihilate the town of which you have spoken. (22) Hurry, flee there, for I cannot do anything until you arrive there.” Hence the town came to be called Tzoar. (23) As the sun rose upon the earth and Lot entered Tzoar, (24) God rained upon Sodom and Gomorrah sulphurous fire from God out of heaven. (25) God annihilated those cities and the entire Plain, and all the inhabitants of the cities and the vegetation of the ground. (26) Lot’s wife looked back, and she thereupon turned into a pillar of salt.

(27) Next morning, Abraham hurried to the place where he had stood before God, (28) and, looking down toward Sodom and Gomorrah and all the land of the Plain, he saw the smoke of the land rising like the smoke of a kiln. (29) Thus it was that, when God destroyed the cities of the Plain and annihilated the cities where Lot dwelt, God was mindful of Abraham and removed Lot from the midst of the upheaval.

ותהי נציב מלח AND SHE BECAME A PILLAR OF SALT — By salt had she sinned and by salt was she punished. He (Lot) said to her once: “Give a little salt to these strangers” and she answered him, “Do you mean to introduce this bad custom, also, into our city?” (Genesis Rabbah 50:4).

Don't look behind you - Because Lot and his wife were pained that they weren't also able to save all their property, therefore it sayd "his wife looked behind him and became a pillar of salt". It should have said to look 'behind her'.

And if it is according to the words of Rashi z"l, that she was made into a pillar of salt because she sinned with salt and did not give it to the poor, it is difficult. Why was she punished specifically in this moment?

Rather it's now that she pained by the loss of her possessions, and looks to what will be after the death of her husband. That's what it means when it says "behind (after) him". For she said, that her husband won't leave her anything after he's gone, and she won't have any way to claim her ketubah from him.

Therefore this glance caused the remembrance of her sin, for the divine aspect of judgement prosecuted her saying "it's not enough that from your money you didn't give charity" so she was given over to salt on the principle of "there is no salt (to preserve) money" (ketubot 66b). And her eyes were also narrow when it came to guests, because the text says "he baked unleaven bread", implying that he did but not her.

And if so, the sin his wife sinned was that in this great upheaval, she was concerned about her possessions, not in order to do charity, for that wasn't her way. Therefore she was punished, and became a pillar of salt, based on the idea "there is no salt (to preserve) money". She sinned and was punished with salt...

She said to him: My teacher, sustain me. He did not recognize her, so he said to her: My daughter, who are you? She said to him: I am the daughter of Nakdimon ben Guryon. He said to her: My daughter, the money of your father’s household, where did it go? How did you become so poor? She said to him: My teacher, is it not that they say such a proverb in Jerusalem: Salt for money is lacking [ḥaser]? There is nothing with which to preserve it and prevent it from being lost. And some say the proverb asserts that kindness [ḥesed] is salt for money, i.e., using money for acts of kindness preserves it. He continued to ask her: And the money of your father-in-law’s house, which was used properly, for benevolent acts, where is it? She said to him: This one came and destroyed that one; all the money was combined, and it was all lost together.