Source Code Only Allowed With One Open Source Project

Question

I have an open source project that is licensed under the GNU General Public License v3 and it's done in Windows Presentation Foundation. I came across someone that sells themes/skins for WPF projects and he is willing to donate one of the skins for use with open source project. However, he only wants the skin to be allowed to be used with my project. I was wondering what the best way would be to prevent other people from stealing the skin and using it with their projects? Is there a license that would fit this requirement? Or is there a better way to accomplish this?