Hi nm and welcome to the forum
Your house sale cannot be deemed to be a windfall, as the property is already included in the IVA proposal - evidenced by the fourth year revaluation. Who has told you the sale would represent a windfall?
It is possible for you to offer a variation to the terms of your arrangement, providing that this gives the creditors at least the dividend that they originally expected. This will need to include your husband's half share of the equity raised from the sale, and possibly some of yours as well to make up for the future IVA contributions.
Ask your IP to provide you with a settlement figure, to see whether your idea is feasible.
Regards, Melanie Giles, Insolvency Practitioner for over 20 years.
For further details contact me at
http://www.melaniegiles.com and view my IVA blog at:
http://melaniegiles.blogs.iva.co.uk