The relationship between pshat and derash is exceedingly complex and the definitions of these terms are subject of intense debate. As late Professor Nechama Liebowitz was wont to comment: 'It is peshat if I say it but it is derash if you say it.'
Some claim that peshat is that interpretation which most successfully integrates and explains both the local and distant context. In other words, peshat must be able to take into account parallel usages, linguistic and historical data, and common interpretative sense. Derash, on the other hand, is satisfied to successfully explain a phrase or a string of words, with no concern for resultant contradictions, even nearby ones.
While this definition has some validity, it must be broadened to include theological and moral concerns. Many texts resist categorization. One must remember that frequently each interpretation resolves some issues but leaves others less than optimally resolved. Not infrequently the "simplest" explanation conflicts with distant context in which I include the theological or moral perspectives held by tradition. It is no more objectionable to seek to re-interpret in such a case than it would be had there be a linguistic, historical or scientific contradiction. In such situations, the Sages may elect to offer an explanation that they would undoubtedly term peshat even if others may consider it a derash. They choose a trade-off between the simplest and the most theologically and morally correct interpretation. The point is that neither is all-inclusive and each gives something up but in return receives something else. The Sages may choose to be right with the Hashkafa of Judaism and wrong in purely interpretative sphere rather than choosing the peshat that contradicts received teaching. We have an example of this phenomenon in our sedra:
And HaShem spoke unto Moses, saying:
Speak unto the children of Israel: When a man or woman shall commit any sin that men commit, to commit a trespass against HaShem, and that soul be guilty;
then they shall confess their sin which they have done; and he shall make restitution for his guilt in full, and add unto it the fifth part thereof, and give it unto him in respect of whom he hath been guilty.
But if the man have no redeemer, the restitution for guilt which is made shall be HaShem's, even the priest's; besides the ram of the atonement, whereby atonement shall be made for him.
The verse appears to be saying that the restitution should be made to the injured party but if he had died and no relatives can be located, it should be re-directed to the priests. This explanation is especially reasonable in light of parallel verse in Vayikra 25:26 where that is what it means.
If thy brother be waxen poor, and sell some of his possession, then shall his kinsman that is next unto him come, and shall redeem that which his brother hath sold.
And if a man have no redeemer, and he be waxen rich and find sufficient means to redeem it;
then let him count the years of the sale thereof, and restore the overplus unto the man to whom he sold it; and he shall return unto his possession.
But if he have not sufficient means to get it back for himself, then that which he hath sold shall remain in the hand of him that hath bought it until the year of jubilee; and in the jubilee it shall go out, and he shall return unto his possession.
It is surprising, therefore to read the following comment in the Sifre (Naso 4):
R. Ishmael says: Is there any person in Israel who has no redeemer? What means "no redeemer"? It came to say teach about one stealing from a convert (who halachically has no relatives) and he swore to him (denying his responsibility to pay) and the convert dies. He must pay the principal and the fifth to the Kohanim...
"...the restitution for guilt which is made shall be HaShem's, even the priest's" - Hashem acquired it and gave it over to a Kohen.
R. Ishmael interprets the verse against the sister verse in Vayikra. He says that it refers solely to a case of a convert who had died and that "no redeemer", means an ontological absence of relatives. How may one explain this convoluted interpretations that is against textual precedent?
The key lies in the second part of the verse: "... the restitution for guilt which is made shall be HaShem's, even the priest's; besides the ram of the atonement, whereby atonement shall be made for him." The Rabbis must have been bothered by the question of how and why the restitution becomes due to the priests, if it speaks simply of the case where a redeemer was not located. Besides the abuses that such a system invites, how can it be properly administered? How long, for example, must one wait for a "redeemer" to arrive and how and who would seek him out? There seems to be no precedent in Mosaic legislation for property due to an individual to be transferred to the priests.
Faced with this difficulty, Chazal fall back on their understanding of the rules of inheritance and family relationships and rights. The Rabbis saw a convert as leaving behind his or her own people and family to become a direct member of G-d's own family. One who corrupts the judgment of a convert is as if he corrupted the Judgment of Heaven (Chagiga 5). In a way of speaking, a convert becomes a child of G-d; as such, G-d naturally becomes his or her heir. So, also the Kohanim. They also have a familial relationship with G-d, who they represent.
This idea is intimated already in the book of Ruth (2:11-12).
And Boaz answered and said unto her: 'It hath fully been told me, all that thou hast done unto thy mother-in-law since the death of thy husband; and how thou hast left thy father and thy mother, and the land of thy nativity, and art come unto a people that thou knew not heretofore.
HaShem recompense thy work, and be thy reward complete from HaShem, the G-d of Israel, under whose wings thou art come to take refuge.
The expression, "refuge under His wings" is also the one used regarding marriage as in Ruth 3:9: "And she answered: 'I am Ruth thine handmaid; spread therefore thy wing over thy handmaid; for thou art a redeemer'."
Note that this approach explains the 2nd part of the sentence very well while the more superficial approach leaves it conceptually enigmatic. One, therefore finds that commentators such as Ibn Ezra who did not explain according to this midrash, had to address this issue to offer and explanation for it.
In summary, we encounter here a not uncommon situation where an apparent derash turns out to be an attempts to deal deeply with exegetical difficulties inherent in the text on the basis of an otherwise well established moral and theological tradition.
1 Interestingly it reappears in regards to conversion in passages such as this one: And many nations and great societies will come to look for the G-d of Hosts in Jerusalem and to seek G-d's favor. This is what the G-d of Hosts says, during those days 10 people of all of the foreign tongue will grab onto the wing of a Jew's garment and say let us come with you because we have heard that G-d is with you. (Zechariah 8-22,23) See also the comments of Baal Haturim to Shemos 21:8.