Is my dog going to be okay?

For a Christmas gift for my friend I was covering a piece of very burnt toast with varnish. My mom was helping me with it, and we were working on it outside. She let my dog go outside, unsupervised, because she didn't think that my dog would eat it. Well i go out to check to see if it has dried any and found that the piece of toast was gone, therefore thinking that my dog might have eaten it. There are other possibilities like her just carrying it to another part of the yard and not eating it (it was dark out so we couldn't see) but that is probably unlikely. How dangerous is varnish for a dog? I''m not sure if any veterinarian offices in our area are open because of it being late and the holidays so I don't know if taking her there is an option. I'm scared and don't want my poor dog to get really sick before Christmas. By the way, she's 5, about 60 pounds, and a Golden Retriever if that helps any. Thanks.