In order to sell that single night, you need to go to your Listing --> Edit --> Availability and set that one night as a minimum of one night, which over-rides your standard minimum.
I hope they never sell single nights without my exlicit permission! My minimum is more than this, and I often enjoy that single night off, planning an evening with friends. But, I don't want to block single nights on my calendar so that my current or next guest know that they could extend their existing reservation if their plans change.