Hi sb and welcome to the forum
This will depend upon the actual terms of your IVA, but assuming you have the usual 50% uplift modification you should only need to pay half of your payrise for now.
At the end of the year, however, your IP should revisit your income and expenditure and set a new payment amount based upon your earnings less your expenditure. This revised figure then becomes the new benchmark for any further 50% uplifts - so in theory you could end up paying more.
The aim must always be to pay as much as you can afford to creditors, and if this means that your IVA will be repaid earlier then this has definate advantages for you as well.
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