You can download RedHat 8.0 directly from RedHat's site without paying a cent and install that copy of RedHat on all 5,000 desktop systems. You might want to burn a few copies of the CD or set up an install server as it might take you some time to install it 5,000 times serially. Perfectly legal and within your rights.
Now if you want support from RedHat if you have a problem, you might want to purchase a few copies or work out some sort of support agreement with them that will cost you. But if you don't need direct support you can use if for free all you want. Remember, you're in a GNU world. No more proprietary money sucking grubs.