Sometimes you make a mistake in creating a budget for your project when you propose it. Doesn't matter who you sent it to, public agency or private company, sometimes you make a mistake. Most agencies will let you move money around between buckets within the grant as long as it serves the purposes of the program and you can justify it. Lots of agencies won't let you buy a computer unless you swear on a stack of your mother's graves that you're going to use it 100% for the project and nothing else. They might not check, but they might make you swear anyway, and your university will be looking as well. Companies are similar, though maybe not as caring about the details. Everything, as Jon points out, depends on the terms of the agreement between your university and the granting entity. Lots of corporate money comes in as an unrestricted gift with no strings, whereas virtually no public funds have no strings.
Read the terms.