Praise be to Allah.
You should understand that being prudent and keeping away from that which is dubious, and always trying to be on the safe side with regard to matters of halaal and haraam, is a wise approach and proper conduct, which is enjoined by Islamic teaching. The Prophet (blessings and peace of Allah be upon him) said: “Whoever guards against the doubtful matters will protect his religious commitment and his honour, but whoever falls into that which is doubtful will fall into that which is haraam, like a shepherd who grazes his flock around prohibited land; he will soon graze in it.”
Narrated by al-Bukhaari (52) and Muslim (1599).
And he (blessings and peace of Allah be upon him) said: “Leave that which make you doubt for that which does not make you doubt.” Narrated by at-Tirmidhi (2518) and an-Nasaa’i (5711), at-Tirmidhi said: This is a hasan saheeh hadith. It was classed as saheeh by al-Albaani in Saheeh at-Tirmidhi.
‘Umar (may Allah be pleased with him) said: We used to refrain from nine tenths of that which is halaal for fear of falling into that which is haraam.
Abu’d-Darda’ (may Allah be pleased with him) said: Part of perfect fear of Allah (taqwa) is caution regarding an atom’s weight, to such an extent that he will refrain from some of that which he thinks is halaal for fear that it may be haraam, so that it will form a shield between him and the Fire.
What appears to be the case from what you say is that your father is giving you a certain amount of money to cover your study expenses, and so long as he is doing that, you should use this money for the purpose of study. If there is anything left over from it, then you must tell your father that some of this money is left over. Then if he asks you to return the extra amount to him, you have to return it to him, but if he lets you off, then this money is halaal for you.
The ruling is like that because what appears to be the case is that your father is only giving you this money as a gift on condition that you use it to pay for your studies. If a gift has a condition attached to it, then attention must be paid to the conditions stipulated by the giver. It says in Asna’l-Mataalib (2/479) by Shaykh Zakariyya al-Ansaari (may Allah have mercy on him): If he gives him some money and says “Buy yourself some headgear with it, or go to the public bath” and the like, then he should dispose of it in that manner only, in accordance with the wishes of the giver, whether he intended that he should cover his head with the headgear or clean himself by going to the public bath, because of what he saw of him having his head uncovered or looking scruffy and dirty. But if he did not intend that, and he only said it by way of general talk, then he does not have to use it for that purpose; rather it becomes his property and he may dispose of it however he wishes. End quote.
Shaykh Sulaymaan ibn ‘Umar al-Jamal (may Allah have mercy on him) said: If he gives him dates with which to break his fast, then he should use them for that specific purpose, and it is not permissible for him to use them for any other purpose, based on the intention of the giver.
End quote from Haashiyat al-Jamal ‘ala Sharh al-Manhaj (2/328)
If your father did not mind about leaving this extra money with you and you have siblings, then he must give your siblings the equivalent of the extra money that he gave to you, because the most correct scholarly opinion is that it is haraam to differentiate between one’s children (with regard to gift giving) unless there is a valid reason for singling out one of them and giving him more, as we have explained in fatwa no. 178463
And Allah knows best.