Acts 24:17 reads:
"Now after some years I came to bring alms to my nation and to offer sacrifices" (NRSV)
Now, Downs (correctly) argues that the bringing of alms here sounds more like an act of private piety than the delivery of the collection that Paul had organized from the churches of Achaia and Macedonia ("Paul’s Collection and the Book of Acts Revisited" NTS 52 2006 p50-70). Luke's Paul is here presenting the collection as an act of private piety. Why did he do so? Well, I argued in my last blog post that the collection had been prohibited by the Jews of Achaia, who had jurisdiction in such matters, and that it was therefore illegal or at least controversial. It seems to me that this neatly explains why Paul presents the collection as an uncontroversial act of private piety. Paul, on trial for his life, choses his words carefully, preempting any accusation his accusers might bring about the collection.
Luke did not want to draw attention to the fact that the Christians had defied authority, so he did not reveal that Paul had collected funds from Galatia and later from Macedonia and Achaia. He was comfortable to mention only the uncontroversial famine relief by the church of Antioch (Acts 11:27-30) and Paul's carefully chosen words about bringing alms to Jerusalem. I suspect that Luke was cautious because adversaries of the church could get hold of a copy of Acts. Luke's intended audience, on the other hand, may have already known about the collection, in which case they would have understood the significance of Acts 24:17.
Apart from Acts 24:17, do we have other examples of cases where Luke's Paul carefully choses his words to save his skin? I would like to bring up one example. In Acts 22:1-3 Paul stresses his own strict Jewish credentials, and upbringing at the feet of Gamaliel. Luke here is not making the claim that Gamaliel was Paul's only teacher, or even his main teacher, as many suppose. Under the circumstances it would be legitimate for Paul to be selective with the facts. Remember that this is Luke's account of Paul's words, not Luke's account of events. Then, at Acts 22:12-15 Paul gives not hint that Ananias was a Christian, but says that he was devout according to the Law and respected by the Jews, and that he had endorsed Paul's future work. This information is completely absent from Luke's account of the same events in Acts 9:10-17. In Acts 22:12-15 Paul is seeking support from his audience by claiming that his work had the backing of a devout Jew. Luke's Paul, in great danger, understandably chooses his words carefully to try to win over his hearers.
We cannot, of course, prove that Acts 24:17 and Acts 22:1-3,12-15 are not Lucan invention, but it seems to me that these claims to Jewish piety are just the kinds of things that we would expect Paul to say, given the circumstances.