How to Spend Money Dad Gave for a Specific Purpose
Question

My father gives me some money to use for a specific purpose, without emphasising that it is a trust and is only to be used for that purpose, or telling me not to use it for any other purpose. But sometimes, when I need money but do not have any, I use that money that he gave me to buy what I need. What is the ruling on that? Is the food that I buy with that money regarded as prohibited? Please note that he does not know about that.
Praise be to Allah.Whoever is given something to use for a specific purpose is not allowed to use it for anything else, except with permission, because it is a gift with restrictions, so the restrictions should be taken into account, unless the recipient knows that the one who gave him that gift meant it to be used in this way but he would not mind if it is used in some other way.
It says in Asna Al-Matalib (2/479) by Shaykh Zakariyya Al-Ansari (may Allah have mercy on him):
“If he gives him money and says: Buy yourself a turban with it, or go to the public bath, and the like, then it must be used for that purpose, as the purpose was specified by the giver.
This applies if he intended that he should cover his head with the turban, or clean himself by going to the public bath, because he saw him bare-headed and looking unkempt and dirty.
Otherwise, if he did not mean that specific purpose – rather he said it by way of normal chatting – then it does not have to be spent in that particular way, and the recipient may keep it and spend it however he wishes.”
Shaykh `Ulaysh Al-Maliki (may Allah have mercy on him) said:
“If a group of people or an individual help a Mukatab [a slave with a contract of manumission] to pay off that contract by giving him money and he paid it off, and there is some money left over, then if the helpers did not intend to give the money as charity to the Mukatab – rather they intended only to free him from slavery, or they did not have any particular intention in mind – in that case, the helpers may take back the surplus amount, if they wish, and share it among themselves. Or, if the Mukatab is unable to manumit himself after giving their money to the master, they can go to the master and ask him for what this slave gave him of their money.
But if their aim was to give as charity the money that they gave to the Mukatab to help him, then they cannot take back the surplus or ask the master for it if the slave was unable to manumit himself.
If some people help the Mukatab with his contract of manumission by giving him money, then he pays off that contract and there is some money left over, then if they helped him to free himself from slavery, not by way of charity, he should return the surplus to them, to be divided proportionately among them based on their original contributions, or they may let him off. If he is unable to pay off his contract of manumission, then everything that his master received from him before it turned out that he was unable to pay off his contract is permissible for the master, on the grounds that it was the slave’s earnings or charity that was given to him.
But if he was helped for the purpose of freeing himself from slavery, and the money is not enough to pay off his contract of manumission, then everyone who tried to help him may take back what he gave, unless the Mukatab is let off [by the people who gave him money], in which case the money is his to keep. But if they gave him money as charity, not for the purpose of helping him to free himself, then his master keeps the money, if the money is not sufficient to manumit him.”
Al-Juzuli said: “If someone is given money for a particular reason , such as for the knowledge that he has, or for being righteous or for being poor, when he is not like that, then he must not accept it, and if he does accept it, he must return it. It is prohibited for him to consume it, because in that case he is consuming ill-gotten wealth." (Minah Al-Jalil  9/475).            
If your father’s aim was to achieve a specific purpose, then you do not have the right to spend the money on anything else, even if he did not emphasise that it was a trust and was only to be used for that purpose.
But if he was only advising you [as to how it was to be spent] and he would not mind if you spend it on something else, then there is no blame on you in that case.
Therefore you should be careful when spending that money, and you should not spend it on anything that you know your father would object to, or you are not sure whether he would approve or not.
If you are not sure about something, then you must only spend it on the purpose for which he gave it to you, or ask him and seek his permission .
For more details, please read the following answers: 191708 , 148469 , 21701 , and 42220 .
And Allah knows best.