Is it permissible for me to invest the property of an orphan under my care so that he’ll get the profits later?
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
Ibn Manzur cited the meaning of an orphan from Ibn Barriy: “Al-Yatim (الْيَتِيمُ) is a person whose father died. Al-‘Ajiy (العَجِيُّ) is a person whose mother died while al-Latim (اللَّطيم) is for a person whose both parents died.” 
Imam al-Jurjani in his book al-Ta’rifat: An orphan is a person whose father died because his sustenance and welfare is the responsibility of the father and not the mother. While for animals, an orphan is the one its mother died because milk and food are obtained from its mother. 
Thus, it can be understood that an orphan refers to a child of whom his father died. Ans the child hasn’t reached puberty. In a hadith narrated by Ali bin Abi Talib RA, the Prophet PBUH said:
لاَ يُتْمَ بَعْدَ احْتِلاَمٍ
“No orphan hood after the age of puberty,” 
Syeikh Syams al-Haq al-‘Azim Abadi cited the statement of Ibn Ruslan who said: When an orphan regardless of whether a boy or a girl reaches puberty, where they dream like most people, then he/she is no longer considered as an orphan and the rulings of an adult apply. 
Taking care of the property of an orphan is placed under the responsibility of the wali (legal guardian) of a chosen and appointed person by a judge. Every wali chosen is obligated to ensure the property of the orphan is safe and managed properly before it is returned to the orphan. Allah SWT states:
وَلَا تُؤْتُوا السُّفَهَاءَ أَمْوَالَكُمُ الَّتِي جَعَلَ اللَّـهُ لَكُمْ قِيَامًا وَارْزُقُوهُمْ فِيهَا وَاكْسُوهُمْ وَقُولُوا لَهُمْ قَوْلًا مَّعْرُوفًا
“And do not give the weak-minded your property, which Allah has made a means of sustenance for you, but provide for them with it and clothe them and speak to them words of appropriate kindness.” 
Syeikh al-Maraghi stated that we are restricted from giving property to the weak-minded people (under your care). The property can be given back to the orphans after they have reached adulthood. In this situation, not giving it to them is to protect it from being wasted. Hence, protect and manage the property for them until their thoughts mature. 
Answering the above question, the ruling for a wali to invest or do business using the property of an orphan under his care is permissible. This is based on a hadith of the Prophet PBUH:
اتَّجِروا في مالِ اليتَامَى لا تَأكلُهَا الزكاةُ
“Trade with the property of orphans and then it will not be eaten away by zakat.” 
Syeikh al-Mubarakfuri stated: Imam Malik, Imam Syafie and Imam Ahmad are of the opinion that zakat on orphans’ property is obligatory. Ibn Malik stated the zakat on the orphan’s property will decrease the property gradually. 
The above hadith shows that zakat is obligated on the orphan’s property if it reaches the haul and nisab set. The property should be managed properly so that the welfare of the orphans are taken care of in the future. Thus, to prevent the property from gradually decreasing due to zakat, then it is better for the wali to expand and invest the property or place it in the fixed deposit account or Tabung Haji.
According to the above discussion, we are inclined to state that property that has been divided through faraid owned by an orphan is permissible to be invested by the wali of the property exceed the nisab set of which zakat is obligated for it. This is to prevent the property from gradually decreasing annually due to the zakat imposed on it. However, the wali must guarantee that the property will not disappear or be lost in unprofitable investments. Likewise, if the property’s value is less than the obligatory zakat’s nisab, then it is best to be put in a fixed deposit account or Tabung Haji. Avoid recklessly investing the property of the orphan which will end up with it being squandered.
Hopefully, what is discussed here give an understanding to us all in living our lives in accordance with the true teachings of Islam. Amin.