What is the ruling on vowing to buy an Atari device for younger siblings upon their success in an exam, and what are the implications of not fulfilling the vow after selling the specific device in the same store?

If the questioner made a vow to buy a toy for his siblings if he succeeded, then if the toy includes forbidden matters, it is not permissible to buy it, and the vow is void. If it is permissible, then buying it and vowing it falls under the category of a vow concerning a permissible act. However, if he promised them to buy the toy without making a vow, then he must fulfill the promise, unless the toy includes something forbidden.