I would continue to push back on this and ensure you are satisfied before agreeing to it.
How long is your 'temporary' promotion for?
I trust this increase is only 50% of your increased wages and you are retaining the other 50% ?
Ensure your I&E reflects your true costs and everything is covered.
Sharing from experiences of dealing with debt
There is a solution for everyone .... Just need to stay positive !
I agree with Luluj. If this is a temporary promotion then the 10% 50/50 rule would make more sense and the money can be dealt with monthly. This means no reviews and is very simple. If the promotion is then made permanent it would be advisable to have a full review at this time or at the next anniversary.
You need to speak with the IP and ask them how they arrive at these figures especially in such a short period of time.