It's pretty clear. If you want to show in the exhibition, surely you need to have the work you submit at the time of the exhibition. Easy.
The judges spend their time and effort choosing from the images submitted to them. Not to choose "work that might be like what is submitted" because you never know if the artist's next piece, however similar, is as good. You, the artist, might think so but a judge may not agree.
So put it aside.
Hopefully you are creating plenty of work that putting 1-3 paintings aside for a while won't matter too much. If you MUST sell it (and who wouldn't!?) then good practice is to inform the organisers that your piece is no longer available and you won't be able to participate in the show. In some cases that will look bad and they might remember your name, but in some cases it won't matter.
Be very careful with NFS work. Firstly, make sure it's allowed. Secondly, recognise that rarely are there shows where sales aren't important. Even if they're not the main point of the show it's how organisers cover the costs.