Every dictionary I have checked confirms that 'les soldes' is masculine (wheareas 'la vente' is feminine, and 'la solde' has a different meaning). Just about every French person (woman) I have asked is surprised to find this to be the case ! However, I think the exercise should at least reflect/indicate the 'correct' usage:
Perhaps a simple note under the heading of the lesson that although 'les soldes' is officially masculine gender, it is very commonly 'misused' in the feminine, and throughout this lesson the 'colloquial' feminine grammatical gender for 'les soldes' has been used.
Just leaving it incorrect doesn't seem to me to be the best option.
