Hi.
So first of all, a hamza after an alif is often omitted in colloquial pronunciation and, in result, sometimes often in writing.
So it's not actually الاستبا
but الاستباء
Secondly, it is a maṣdar (a noun derived from a verb, a name of an action) of the VIII (eight) form. It could come from SB' (with ' being the hamza, the apostrophe should be concave on the left, not on the right) SBW or SBY forms.
Only SB' and SBY roots seem to exist.
So the VIII form of the SBY root is the same as the I form in theory, but the VIII form in general is reflexive towards the basic I form, or means doing the I meaning for oneself. The meaning of the I form is to take captive, prisoner (and by extension to captivate, charm, intrigue etc.). VIII form thus literally (the meaning of the form + the meaning of the root) means "to take captive for oneself" or "to captivate oneself", but it means (in the usage mentioned by the dictionaries) to take captive, but also to captivate in a metaphoric sense: to captivate the hearts of someone, like a woman captivating a man.
The VIII form of the SB' root, on another hand, means buying wine to resell it later. This meaning I took from Kazimirski, but Lane mentions the selling of wine as one of the meanings of the I form of the SBY root as well.
Since I don't know the context, I cannot say which of the meanings is fitting. It could be a man from among some group of people taken captive. That the narrator found among the group of captives a righ/good/proper/whatever Christian guy.
But why use the VIII form? I would have been enough.
The preposition there makes it less fitting in my eyes of this being description of him being charming, captivating, because li- preposition would have been used, not min. Even maʿa preposition would do in some cases. But min? It would be strange.
Yours, Maciej Czyż