" "I don't know about that" is an expression we used to say that we do not agree or are very uncertain about something. For me this phrase suggest that B does not want A to have truck 10. It might be his favourite or he might have allocated it to another person.
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84HGaborB: I don't know about that.B is essentially saying, "I'm reluctant to give you what you're requesting" or "I don't think I can do that for you". The exact reason for this kind of response is difficult to infer from the information given in the dialog.