Opportunity at work: All in or hold back and play it safe?

[TheSyntaX]

Long story short: I am a software engineer in IT Health Sector, we provide software to many hospitals around the world.
Today senior dev asked me if I would be okay with taking over his share of work (which is entire new project) and I said yes.

After he checked with our manager - Our manager told him that I am about to be hit with shitload of high priority work from existing clients so that's a no.

I've been working with these clients for the past year, they are considered high priority because they are best and most paying for company, I am very happy and lucky to work with those clients as they are one of the biggest in Australia. I was pretty much assigned to them the moment I joined company due to my experience.

I know I could manage the workload 99% (1% of failing) but I am not sure if I should push for it or just stay dedicated and in safe and just do work I am assigned?
Essentially this would make a difference for my next pay rise (In January), I either get 20% (which I am 100% I will get) or 30% (if I was to over-achieve).

I guess my question is: If anyone had experiences like this - what was your decision and what was the outcome? and what's your tip/suggestion?
Is it even acceptable to ask manager to give you more responsibilities?

[polarbear]

I'd trust your manager's opinion in saying that it might be way too much for one person to handle on top of your regular duties. He was basically asked if it's okay, and his direct response was no. Could be for a very good reason. Why is the senior dev trying to throw all this work on to you anyway?

You're also NOT guaranteed that 30% unless it's in writing. Even if you don't take the risk, you've got a happy 20% raise guaranteed. You could very well be risking a lot of stress that is not your job for absolutely nothing in return. Sounds like you've got a good reputation - there's really no need to fuck that up. I'd stick to your assigned duties.

[TheSyntaX]

The last paragraph of your response killed me.
I rather have solid 20% than risky 30%.

I will just stick to being a "Yes-Man" and complete orders having good reputation for solving any issue than reaching over my head.

Thanks, your response it really helped.