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:
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.