He's not giving up 5% of his future pay. If he's earning 100 now, after the raise it would be 105. He's giving up 5 of that, so 5/105=4.76% or so. Dilbert wouldn't make this mistake. I understand the joke wouldn't work that way, but Scott could have worded it better.
That said, giving up 4.76% of your pay to halve your work sounds very smart.
The big mistake here is that in every corporation, "suggestions" like this from the PHB always have an implied "do it or you're fired" at the end. Dilbert's choice is not 200% work for 105% pay vs. the status quo (or 100% work for 95% pay), but in fact 200% work for 100% pay vs. 0 work for 0 pay.