Rav Ari Bensoussan quotes the Alter of Navardok who says that when someone doesn't utilize their talents to accomplish a particular mission Hashem designed for them, then anyone can take the unfulfilled mission and receive tremendous siyata d'shmaya to accomplish the task even though it is beyond that individual's abilities. What is the earliest source for such a concept?
Does this gemara in Chagiga 15a Sefaria's translation help you?
שאל אחר את ר"מ לאחר שיצא לתרבות רעה א"ל מאי דכתיב (קהלת ז, יד) גם את זה לעומת זה עשה האלהים אמר לו כל מה שברא הקב"ה ברא כנגדו ברא הרים ברא גבעות ברא ימים ברא נהרות
The Gemara relates: Aḥer asked Rabbi Meir a question, after he had gone astray. He said to him: What is the meaning of that which is written: “God has made even the one as well as the other” (Ecclesiastes 7:14)? Rabbi Meir said to him: Everything that the Holy One, Blessed be He, created, He created a similar creation corresponding to it. He created mountains, He created hills; He created seas, He created rivers.
אמר לו ר"ע רבך לא אמר כך אלא ברא צדיקים ברא רשעים ברא גן עדן ברא גיהנם כל אחד ואחד יש לו ב' חלקים אחד בגן עדן ואחד בגיהנם זכה צדיק נטל חלקו וחלק חברו בגן עדן נתחייב רשע נטל חלקו וחלק חברו בגיהנם
Aḥer said to him: Rabbi Akiva, your teacher, did not say so, but explained the verse as follows: Everything has its opposite: He created the righteous, He created the wicked; He created the Garden of Eden, He created Gehenna. Each and every person has two portions, one in the Garden of Eden and one in Gehenna. If he merits it, by becoming righteous, he takes his portion and the portion of his wicked colleague in the Garden of Eden; if he is found culpable by becoming wicked, he takes his portion and the portion of his colleague in Gehenna.