Why is Kalev seeking someone to conquer Kiryas Sefer? He was given only Chevron!
Maharsha (Temurah 16a): This is why Chachamim explained this as parables (refer to 1:11:1:1). Also, it is because it was already written in Yehoshua, Perek 15 (also Malbim verse 11 says so).
How could Kalev promise his daughter to the one who will conquer the city? Perhaps it will be someone forbidden to marry into Yisrael, e.g. a Mamzer!
Vayikra Rabah 37:4: Indeed, Kalev erred. Even so, Hashem sent him Osni'el, a most worthy man to conquer the city and marry Ochsah.
R. Chaim Paltiel (Bereishis 24:14): Ta'anis 4a does not list Kalev among those who asked improperly, for "conquering the city" alludes to recovering the Halachos that were forgotten in the mourning over Moshe. Surely a Mamzer would not merit this! 1
Even though a Mamzer can be a Chacham (Horiyos 13a), surely he would not merit such an awesome level of Chachmah! (PF)