What is the ruling on buying something with money from one's father, without intending to buy that specific item, given that the father swore by triple divorce upon the mother if he gives the questioner money to dispose of it with no intention of buying that item?

You must avoid buying the item sworn upon by triple divorce, because the apparent meaning of the oath is that the father intends to forbid all purchasing. Swearing by divorce is a conditional divorce according to the majority of scholars. If you buy the aforementioned item, you would have made your mother unlawful for your father. Some scholars are of the view that swearing by divorce has two situations: either it is a conditional divorce, in which case it occurs, or it is intended as an oath only, in which case it is an oath that can be expiated. Scholars differ on the occurrence of triple divorce with a single utterance, and the view of the majority is that it occurs.