@michael.pearce, right, this is hard to explain. For everyone’s benefit, I’ll actually rewind and break it down more (sorry, because this got very long):
The indirect object (and the indirect object pronoun that replaces it) is not an element that truly indicates possession of something. It just tells us who is the ‘recipient’ of the action described by the verb (to whom, from whom, or for whom the action of the verb is intended).
His mother gave him life.
= A sua mãe deu-lhe a vida.
The verb gave makes us ask “gave what?”. That’s the direct object: life. To whom was it given? That’s the indirect object: (to) him. Note the unrelated possessive telling whose mother we are talking about.
His mother gave him his life.
= A sua mãe deu-lhe a sua vida.
Here is where we actually have proper possession of life being expressed, by “a sua vida”. Also, we have both the indirect object pronoun and a possessive in the same portion of the sentence, to further illustrate that they’re two complementary, but separate things.
With the examples from the other post, what you might have noticed is that we can sometimes omit one of these two elements when we can easily infer the omitted part or when it’s not our focus.
- Os chefes despediram o Rui pois roubava-lhes documentos importantes.
The second half just says that he “stole important documents from them” (indirect object). There’s no obvious possessive because we either assume that the documents were theirs or we’re just not so focused on who the documents belong or refer to.
Similarly, we can write:
- Os chefes despediram o Rui pois roubava documentos importantes deles.
Here, it says that he “stole their important documents”. But stole from who exactly? From the bosses’ own hands, from the assistant that was carrying them around…? There’s no obvious indirect object here (for this verb, in European Portuguese, we usually structure it as roubar a, not roubar de), but we can still do without it.
@mrchasi’s post and my own may have given the impression that these two sentences are 100% interchangeable or that the two elements are mutually exclusive, but that’s not truly the case. We could just as well write the whole thing and each element would add its own bit of information:
- Os chefes despediram o Rui pois roubava-lhes documentos importantes deles.
On my previous post, I wasn’t so focused on indirect object pronouns, but on the fact that we have two types of possessives at our disposal, where one type (dele, dela, deles, delas) helps to clear out the ambiguity caused by the other (seu, sua, seus, suas). So, it’s not about dropping the indirect object pronoun to solve the ambiguity; it’s about switching from one possessive to another to reduce possessive ambiguity, whether or not indirect-object ambiguity is still there.
- Os chefes despediram o Rui pois roubava-lhes os seus documentos importantes (who’s “seus”?)
=> Os chefes despediram o Rui pois roubava-lhes os documentos importantes deles (ahh, that’s who “seus” refers to)