Your translation is basically right (with some allowance for paraphrase), except for eis rebus.
cum here is circumstantial, and might be translated as adversative , “although”, and embedded in the result clause.
He was a man of such generosity that, although he owned farms and gardens in a number of places, he never placed guards in them to watch over the produce, so that no one would be prevented from enjoying the things each one wanted [to enjoy].
. . . quominus eis rebus quibus quisque vellet frueretur.
eis rebus (which you translated “on his property”) is not locative–it’s the complement of frueretur and the antecedent of quibus: "enjoying those things which . . . "
Grammatically there are two possible analyses of this phrase. Volo can take either (1) an infinitive complement (“to want to do something”) or (2) a direct object (“to want something”).
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Understand frui with vellet (infinitive complement of vellet): quominus eis rebus quibus quisque [frui] vellet frueretur. “[so that no one would be prevented from] enjoying [quin frueretur] those things which anyone wanted [to enjoy].” Here quibus is the ablative complement of understood frui, which is unnecessary in proximity to frueretur and might sound redundant or otiose or inelegant.
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A second interpretation: quibus is the direct object of vellet. Normally the direct object of volo would of course be accusative. However, sometimes (indeed, often) a relative pronoun is “attracted” into the case of its antecedent. This construction is explained (not much better than I have) at Allen & Greenough 306a:
A relative occasionally agrees with its antecedent in case (by attraction):—
sī aliquid agās eōrum quōrum cōnsuēstī; (Fam. 5.14), if you should do something of what you are used to do. [For eōrum quae .]
http://www.perseus.tufts.edu/hopper/text?doc=AG+306&fromdoc=Perseus%3Atext%3A1999.04.0001
Thus, in this passage quibus could be explained as having been “attracted” into the case of eis rebus, which is the ablative complement of frueretur, instead of eis rebus quas vellet frueretur.
I don’t think there’s much reason to prefer either analysis here–take your pick. I think I’m inclined towards #1, only because I feel more comfortable with an infinitive complement of vellet, and also because inserting frui in the neighborhood of frueretur would be inelegant. Maybe C.N. didn’t think too hard about this–he just wrote what came naturally.