Commentary
This verse has appeared a little earlier (36) almost in the same words. As obvious, the same thing has been mentioned here, however, it has a difference. At this place, there is an addition of: مِنْ عِبَادِهِ (out of His servants) after: مَن يَشَاءُ whomsoever He wills) and: لَہ، (lahu: for him or whomever) after: یَقدِر (yaqdir: straitens). From the statement: مِنْ عِبَادِهِ (min 'ibadihi: from His servants), it is gathered that this rule of guidance has been put forth for His particular servants, that is, for the believers, and the purpose is to alert people of faith that they should not start loving wealth and comfort to the extent that their hearts choke when it comes to spending at occasions and on rights enjoined by Allah Ta’ ala. As for the earlier verse (36) that carries the same text, it was addressed to disbelievers and polytheists who prided on the worldly assets of wealth and children and declared these to be the proof of their success in the Hereafter. Thus, any discordance between the addressee and the purpose of address stands eliminated. Maulana Ashraf ` Ali Thanavi (رح) ، in his khulasah of Tafsir Bayn ul-Qur'an, has taken the same approach by first adding 'the believers' in parenthesis while explaining this verse.
Another difference between these two verses pointed to by some commentators is that mentioned in the first verse was the distribution of sustenance between different human beings, that is, Allah Ta’ ala gives more of wealth and property to some, and less to some others - all in His wisdom and in the light of universal considerations. And in this verse, only one person and his different states have been mentioned, that is, this one person has, at times, more with him, then, comes another time and the same person has much less as well. The word: لَہ (lahu: for him) which appears in this verse after: یَقدِر (yaqdiru: straitens) releases an indication in this direction. This approach too leaves no discordance behind. Rather, the first verse turns out as relating to different individuals and the present verse, to different states of one single person.
The sentence: وَمَا أَنفَقْتُم مِّن شَيْءٍ فَهُوَ يُخْلِفُهُ (And whatever thing you spend, He replaces it. And He is the best of the sustainers.) in verse 39 literally means: 'For anything that you spend, Allah Ta’ ala gives you a return from His unseen treasures - at times, within this world and at times, in the Hereafter and at times, in both.' In things happening around us, we see that water comes down from above, animals and humans use it freely, needs of farms and forests are satiated, and no sooner does that supply of water gets used up, than another supply descends to replenish it. Similar is the case of wells dug for water that, no matter how much water is spent out of these, it stands replenished by nature from other sources of water beneath the bed. Man eats up his food leaving the impression that he has finished it, but Allah Ta’ ala provides him with other food in its absence. Physical exercise burns out calories of food and other workings of nature turn it into energy. In short, whatever man spends out in this world, it is the customary practice of Allah Ta’ ala that He would replace it with something else similar to it. Something happening contrary to this, as an exceptional case, - either to punish one, or for the sake of some other creational consideration - will not be deemed as contrary to this customary Divine practice.
According to a Hadith of Sayyidna Abu Hurairah ؓ in Sahih of Muslim, the Holy Prophet ﷺ said, "Everyday when people rise to see another morning, two angels descend from the heavens and pray: اللَّھُم اَعطِ منفقاً خَلفاً واعط ممسکاً تلفق (0 Allah, bless the one who spends [ out of what You gave him or her ] with its return, and let the one who withholds [ what You gave him or her ] find it wasted." And according to another Hadith, the Holy Prophet ﷺ said, "Allah Ta’ ala has told me: You spend on people, I shall spend on you."
There is no promise of a return for spending that is not in accordance with the Shari'ah
Says a Hadith of Sayyidna Jabir ؓ see that the Holy Prophet ﷺ said, "Good deed is sadaqah (an act of charity in the way of Allah). When someone spends on himself or his children and family, that spending too falls under sadaqah. It brings thawab (reward from Allah). And one who spends to protect his integrity and honor, this too is sadaqah. And whoever spends whatever he does in obedience to the command of Allah, He has taken it upon Himself that He will give him its return - except that which is spent in (wasteful, extra to need) building or sinful activity, for there is no promise of a return for it."
After hearing this Hadith from Sayyidna Jabir ؓ ، his disciple, Ibn al-Munkadir asked him: 'What is the meaning of spending to protect one's honor?' He said, 'There may be a person about whom one apprehends that, should he not give him something, he would go about maligning him in all sorts of ways. In this case, giving to such a person is in order to save one's honor.' (Reported by ad-Darqutni, Qurtubi)
With the decrease in the use of something, its production also decreases
The hint embedded in this verse also tells us that as long as the things of use provided by Allah Ta’ ala for consumption of human beings and animals keep being consumed, these keep being replaced by Him constantly. The rule seems to be that the more the consumption of something, the more its production. The multiple uses made of domestic animals like goats, sheep and cows put them high on the list of consumption. They are slaughtered. Their meat is eaten. Then, they are also slaughtered under Islamic legal requirements, such as, the Qurbani قُربَانِی or sacrifice, and in Kaffarat (plural of kaffarah or expiation) and jinayat (faults, offences against religious prohibitions). The more they are consumed, the more increased becomes their frequency of production from Allah Ta’ ala. This is common experience everywhere. The number of these animals, despite being under the knife all the time, remains the highest in the world. The number of dogs and cats is not that high, although the reproduction of dogs and cats should obviously be much more as they produce four or five puppies and kittens in a single pregnancy. A cow or goat delivers two calves or kids at the most. Cows and goats keep being slaughtered all the time. Dogs and cats are (generally) not touched by anyone. But, as far as common observations goes, it cannot be denied that the number of cows, sheep and goats comparatively exceeds the number of dogs and cats. Since the time restrictions have been placed on the slaughter of cows in India, the production of cows has gone down there in that very ratio. Otherwise, every village and every home would have been full of cows that stayed spared from being slaughtered.
Once the Arabs tapered down their use of camels for riding and transport purposes, the usual increase in the populations of their camels has also gone down. Incidentally, what has been said here also helps remove that atheistic doubt usually dished out with reference to the Islamic injunctions of sacrifice saying that it is likely to affect the economy of Muslims adversely.