Hello Rob 71 and welcome to the forum,any surplus in your disposable income would effect your payments into your IVA,this would be decided by your IP,which you would have to notify of any changes,you may be able to keep some of the money for contingencies like moving expenses and decorating,etc.
If your IVA follows the IVA protocol, you will only have to increase your IVA payments by 50% of any increase - so you get to keep half of the saving. Check through your own proposal documents to be sure.
