Musar על מכות 46:12

In our portion the word איה in verse 21 is the allusion to such an event in the future. When the Torah discusses the ערכין and תמורה legislation in Leviticus 27,10 we are told: והיה הוא ותמורתו קודש, (if one did substitute one animal for another) "both the one originally vowed and its substitute will be deemed holy." The word תמורתו can be perceived as symbolizing תמר and ורות whose letters make up the word תמורתו. Both Tamar and Ruth may certainly be viewed as "substitutes," seeing they had not been deliberately chosen for the role they were destined to play. Our sages also say that when Yishai (David's father) slept with David's mother and David was conceived, this was due to an error on Yishai's part. He had thought that he was sleeping with a maid or concubine (Psalms 51,7 Metzudat David). [The verse reads –David speaking – "Indeed I was born with iniquity; with sin my mother conceived me." Ed.] David himself then was also a substitute as far as his mother was concerned. All of these extraordinary circumstances were designed by G–d to demonstrate that the impure is capable of being redeemed and becoming pure. The sperm that impregnated the various mothers mentioned originated in purity, could be called זרע קודש, holy seed. [In a gloss the author draws our attention to Ruth 4,7 where we find the expression על הגאולה ועל התמורה, placing the words "redemption and substitution" side by side. He finds a deep significance in this and speculates that it alerts us to the lesson that through the process of תמורה, substitution, the impure can achieve purity.] Er and Onan died because they defiled such sacred seed. Eventually Yehudah himself had to impregnate Tamar in order to enable the dynasty of David to have its beginnings. When, later on in 38,26, Yehudah acknowledged that he was the father of the infant Tamar was carrying, the Torah adds the word ממני, which our sages in Targum Yerushalmi interpret as a Heavenly voice calling out that the outcome conformed with a Heavenly plan and that neither Yehudah nor Tamar were guilty of misconduct.