Is it true in Islam that: "what goes around comes around?"
If someone does something to you which brings harm or benefit, will the same thing happen to that individual that brought you that harm or benefit?
In the name of Allah, the most compassionate, the most merciful
What a nice matter you mentioned, I indeed appreciate for mentioning that significant important point as you remarked:
what goes around comes around?
I have heard that as well, and strongly believe in that matter too. But the significant related point is that (perhaps) the effect of some or many of our negative or positive activities can be done in the day of Judgement not solely in this world. Besides, this world has not the complete capacity of every reward (positive or negative).
What your remarked (what goes around comes around) is quite true, but it ought to be paid heed that some or many of the harms or benefits will come back to us as well (if they didn't come back to us (directly or indirectly in this world)).
Yes it is true, what goes around comes around whether it is good or bad. Allah (SWT) says in Surat Al-Baqarah about the hypocrites:
وَإِذَا لَقُواْ ٱلَّذِينَ ءَامَنُواْ قَالُوٓاْ ءَامَنَّا وَإِذَا خَلَوۡاْ إِلَىٰ شَيَـٰطِينِهِمۡ قَالُوٓاْ إِنَّا مَعَكُمۡ إِنَّمَا نَحۡنُ مُسۡتَہۡزِءُونَ (١٤) ٱللَّهُ يَسۡتَہۡزِئُ بِہِمۡ وَيَمُدُّهُمۡ فِى طُغۡيَـٰنِهِمۡ يَعۡمَهُونَ (١٥
When they meet those who believe, they say, “We have entered Faith;” but when they are alone with their satans, they say, “Indeed, we are with you; we were only mocking.” (14) It is Allah who mocks at them, and lets them go on wandering blindly in their rebellion. (15)
And again in Surat At-Tawbah:
Among the hypocrites, there are) those who taunt the believers who voluntarily give alms and have nothing but their hard earnings; still they mock at such people. Allah mocks at them, and for them there is a painful punishment.
And for the goodness, take this verse in Surat Ar-Rahman for example:
Is there any reward for goodness other than goodness? (60)
In the Hadith I remember this one, although Imam Al-Albany stated this hadith as weak, but its meaning is still right.
البِرُّ لا يَبْلَى ، وَالِإثْمُ لَا يُنْسَى ، وَالدَّيَّانُ لَا يَمُوتُ ، فَكُن كَمَا شِئتَ ، كَمَا تَدِينُ تُدَانُ
Righteousness does not wear out, and the sin is not forgotten, and the Dayyan (The Judge) does not die, be as you wish, as you condemn, you will be condemned