What about writing a letter or email stating something like you would be happy to provide the food if you are reimbursed, and that your understanding is that there are no other food allergies that need to be accommodated. You could put something in ensuring that you would not be held liable if there are food allergies that you have not been made aware of. In elementary school my mother practically begged to be the one to get the food. It worked out great when they let her. There was one year that they would not let her, and that was the year that I was excluded from a Halloween party. They made me sit outside the classroom with a friend while they had their party. Somehow they thought having me sit outside with a friend rather than alone made it legal. When my mother found out afterwards, heads rolled. And yes, I had a 504.
I would really encourage you to buy the food if you are getting reimbursed and just draft something so that you are not held liable if there are other allergies.