Interesting. Wonder what place it is?
This occasionally happens. Financially, it isn't a problem--they've offered you the money and will pay you. They will likely have to borrow the money from the Dean and have a very small class next year. So integrated over a few years, it won't matter much. It might make it a little harder to get your choice of research advisor. If 12 entering students want to work for Prof. Heisenberg, and Werner only has room for 6, then some people will be out of luck. Generally, though, the smaller classes next year and the year after will even things out.