All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
It is not permissible for you to buy with the money that you save something which your parents (or one of them) prevented you from buying, even if what you intend to buy with it is permissible, because obedience to the parents is obligatory unless they order you to disobey Allah.
You must obey them in what is obligatory, desirable, and permissible, and even in what is disliked according to the view of many scholars, in order to preserve their great right.
Ash-Sharh As-Sagheer reads:
“He (the son) is obliged to obey his parents in what is permissible and what is disliked.” [End of quote]
Besides, Al-Furooq authored by Al-Quraafi reads:
“Al-Ghazali said in Ihyaa’ ‘Uloom Ad-Deen: “The view held by most scholars is that obedience to parents is obligatory in doubtful matters but not in what is forbidden because abandoning what is doubtful is desirable whereas abandoning their obedience is forbidden. Abandoning what is forbidden comes in priority over what achieving what is desirable. He may not travel on a journey to do something or to perform an optional act of worship except with their permission.” [End of quote]
Additionally, An-Nawawi said in Sharh Muslim: “Many scholars held the view that their obedience (parents) is obligatory in matters that are doubtful.” [End of quote]
Moreover, the money that your father gives you is a gift to you, so if he orders you to spend it on a certain thing for an objective he intends to achieve, then it is an obligation to spend the money on it. Indeed, the jurists stated that if the donor of a gift conditions that the money should be spent on something specific for a valid purpose, then it is an obligation to spend it on that.
Zakariyyah Al-Ansaari said in Asna Al-Mataalib, “If a person gives someone money and tells him, ‘Buy a turban with it’, or ‘go to the public bath house’ or the like, then it is an obligation to spend the money on what the donor had specified so as to fulfill the donor's objective.” [End of quote]
To conclude, you should not spend the money on other than what your father or mother had given you permission to spend it on.
Allah knows best.