Just an update - a friendly phone call worked everything out. The musician in question understood the venue's situation once it was explained and we were able to work out a smaller guarantee that the performer could live with, and one that the committee felt comfortable in offering. I feel confident that, unless some unexpected issue comes up (we're booking almost a year in advance - who knows what might happen, such as a competing event, etc) the performer will exceed their guarantee and will be happy with the gig. Somehow in the email exchange the performer got the idea that we were implying they weren't worth a guarantee, and that wasn't the case at all. And the guarantee they were asking for was pretty high, so we were a bit taken aback. All's well that ends well.