How do I cancel dinner when I'm ill after everyone has paid their share?

At noon my brother messaged, "Seven tonight still on?" I had just developed a fever and a cough. The groceries were already in my fridge, bought with the $12 each guest had transferred.

I told him I needed to cancel. He suggested keeping the booking at my flat and letting him do the cooking while I sat at the far end of the table. "Nobody wants to waste the food."

[CDC advises staying home and away from others when respiratory symptoms such as these are not explained by another cause](https://www.cdc.gov/respiratory-viruses/prevention/precautions-when-sick.html). I am keeping guests out of the flat. The question is how to handle the money and disappointment without making illness sound like an excuse for a poorly planned evening.

I can return everyone's share now, but replacing the ingredients for a later dinner would then come from my budget. Is it fair to ask whether people prefer a refund or carrying their contribution over, while making cancellation itself definite?