Why is Yitzchak spelled with a “sin” in Tehillim 105:9?

Question

I just came across this strange variant spelling this morning. I've been in the habit of reciting Tikkun HaKlali recently and saw this in Artscroll's Tehillim this past Shabbat. Thinking it was a typo, I came across it today in a different Tehillim by Artscroll, the Interlinear Edition. The verse reads like so:

אשר כרת את אברהם ושבועתו לישחק

^{(asher carat et av'raham ush'vuato l'yis'chak)}

Normally, of course, Yitzchak is spelled with a tzadde:

יצחק

^(yitz'chak)

Can anyone point me to some commentary on why this is? Both Tehillim which I have used say nothing about the variant spelling, which is interesting to say the least.

Answer 1

In the sefer נפוצות יהודה, the explanation is given as follows:

Yitzchak represents מדת הדין. Since it was this that was responsible for the destruction of the temples and our exiles, the Tanach hints that eventually, מדת הדין will be dropped in favor of מדת הרחמים in the time of redemption. Since the redemption consists of four stages, as alluded to by the four terms of geulah in Shemos 6, and by the four cups drunk during the Pesach seder, therefore, four times in Tanach, the name יצחק is changed to ישחק. (Yermiyahu 33:26, Amos 7:9, 7:16, Tehillim 105:9.) This is to show that eventually, the צ'רות' of the Jews in their exiles will turn around into the ש'שון ו'ש'מחה' of their redemption.

Answer 2

http://www.yba.org.il/show.asp?id=23473