Among the People of the Book are some who, if entrusted with a hoard of gold, will (readily) pay it back; others, who, if entrusted with a single silver coin, will not repay it unless thou constantly stoodest demanding, because, they say, “there is no call on us (to keep faith) with these ignorant (Pagans).” but they tell a lie against Allah, and (well) they know it.
In the previous verses (70-72), breach of faith committed by the people of the Book was mentioned. This included disbelieving in the verses of Allah, confusing the truth with what is false, concealing the
truth and conspiring to mislead Muslim believers.
In the verse cited above (751, their breach of trust in matters of
property has been mentioned. Since some of them happened to be
trustworthy, both types were identified.
Praising good qualities in some non-Muslim is correct
In this verse , some people have been praised for their trustworthiness. If this ‘some’ refers to those among the people of the Book who had embraced Islam, praising them poses no problems. But, in
case, it does not refer to Muslims particularly and instead refers to
the people of the Book in the absolute sense including non-Muslims as
well, then the situation generates the question: When no act of a kafir
(disbeliever) is acceptable why praise them?
The answer is that for an action or deed to become acceptable is a
different matter and having a word of praise for it is something else.
From praising something it does not follow that it is acceptable with
Allah. The purpose is to point out that something good, even if it be
that of a kafir (disbeliever), is good enough in a certain degree, the
benefit of which he receives in the form of good reputation in the mortal world, and of course, in the Hereafter, in the form of a reduction in punishment for his disbelief.
This statement also makes it clear that lslam does not resort to
prejudice and short-sightedness. On the contrary, it shows openhearted appreciation of the excellence of even its adversary in respect of his achievements.
Shafi, Muhammad (2008). Maariful Quran. (Vol .2 surah 3 pg. 98). English-MaarifulQuran-MuftiShafiUsmaniRA-Vol-2.pdf (islamicweblibrary.com)