What is "v'Safek la'Ra'ev Nafshecha", and what is its reward?
Bava Basra 9b: One who comforts an Oni with kind words gets 11 blessings [following this, until the end of verse 12].
Rashi: Console him with nice words.
Radak: These good deeds, e.g. giving bread to the hungry, do with a spirit of generosity and a smile, so he will recognize that you give willingly and with a good heart. The same applies to clothing the naked, which was mentioned (7). V'Safek is taking out, like "v'Yafek Ratzon me'Hashem" (Mishlei 8:35), "Mefikim mi'Zan El Zan" (Tehilim 144:13).
Malbim: The success mentioned until now is for veering from evil - "Pate'ach Chartzuvos Resha" (6), and Tzedakah (giving to the Oni bread, clothing and housing); refraining from these is evil. Now it adds that you should do good amidst love of good and of Hashem. It should be with a full heart and Simchah - give also your Nefesh.
What do we learn from "[v'Nefesh Na'anah] Tasbi'a"?
Radak: The slice of bread that he gives to him should be enough to satiate him.
Malbim: Aside from satiating in his body, satiate also his Nefesh with a spirit of generosity and words of consolation. Then, your reward will be much greater. In place of :Yibaka ka'Shachar Orecha" (8), v'Zarach ba'Choshech Orecha - it will shine suddenly, at once. This is unlike morning, which comes in its time at the end of the night. Rather, amidst darkness, light will shine for you.
What is the comparison to Tzaharayim?
Radak: If you had darkness and affliction, it will return to be like Tzaharayim - the strength of daylight.
Malbim: Even your Afelah, which is darker than Choshech, will illuminate greatly for you like Tzaharayim. It will not be natural, rather, via Hashgachah, miraculous and wondrous, The Nimshal is, aside from the success of the Nefesh that will come at the end of the [physical life, i.e.] night, also in your life a great light and salvation of the Nefesh will shine for you, and you will attain eternal pleasantness also in this world - 'you will see your world in your lifetime' (Brachos 17a).