Is this particular kind of project within the scope of his contract/job description?

@NickZuber We are hired to develop software for our business. Our contracts don't really specify what kind of projects we should work on, but except for the hard maths, it would be a regular C#/F# project like several others on our shop.

You to give some credit for recognizing the difficulty and effort of the task.

If I read correctly, you have other developers. Why not let grumpy do his normal thing and give another person a chance?

I think that asking for a raise each time an employee learn something is a new trend in the IT world. I crossed the path of multiple "grumpy" in the past, and even after a raise, they returned back to the boss nagging another raise for something else. Lot of grumpy history ended up being fired because their attitude started to become bad for the team or their simply gave their notice. I personally think, often, this behavior reflect there is something at home or work that is wrong for grumpy

You said you would pay for the courses, but will you allow him to take the courses and study on paid time? If not, you are asking him to do unpaid work for you.

Are you totally sure that he said this in full seriousness? I also would love the kind of undertaking you're talking about, however, I would have made a joke very similar to what he literally said. I would't have asked a raise but I'd probably say something like: "Ooo, reaching above my pay grade now".

This developer actually sounds like he just doesn't enjoy the mathematics aspect of the project. Hating math is somewhat harmful to being a software engineer, but if we're being honest it's not exactly uncommon. My point being, I wouldn't immediately write this guy off as a slacker with a bad attitude who will never rise in rank. Maybe this particular project just puts him in an uncomfortable and frustrating position.

Learning new things is a core skill in the software development word. We do it constantly, and it's part of the job, not something we get paid extra for. If he doesn't like learning new things, he's in the wrong field, unfortunately for him.

Are you sure you want two programmers, or do you want 1 programmer and 1 statistician? I suspect you need to do some hiring. If you were tasked with coding a robot, would you teach the programmers robotics, or hire an engineer?

What are you asking? How to con the man into selling his work for less than its worth? He doesn't think it's worth his while, either pay him more or find someone else.

I agree with @DavidSchwartz - if courses you wanted him to take would eat additional 10 or 20 hours / week, were you willing to pay him for overtime? And was it clear for him you will? Learning programming skills all the time is the core of our job, learning skills outside of this scope, things that will only be useful for one project is not - so if you wanted him to give you a lot more of his time without paying him a lot more, he might have a fair point. So please, make it clear if these courses would be in his paid time or not, that's important.

@DonBranson "Learning new things is a core skill in the software development word. We do it constantly, and it's part of the job, not something we get paid extra for." Yes, but there are many technologies to learn, some which are interesting to a person, others are not. If I want to learn mobile tech and VR but my employer asks me to learn Fortran, there is a problem

@Mołot: I disagree. At least in my work (which I admit is fairly specialized) I have to be able to learn at least enough about whatever non-programming field I'm doing programs for to understand what I'm supposed to be doing, and why. And I do get paid for that learning time, as part of the job. That said, anyone has the right to ask for a raise any time, for any reason. It's up to the OP, or his managers, to decide whether that raise is justified.

Are you sure that your team's capable of tackling this project? As someone who researches this stuff, I agree that this project sounds exciting - but if your team's just trying something like this for the first time, the learning curve might be a bit much. And since you said that this guy's one of your most experienced developers, he might realize that. You might want to ask him for perspective on this project's viability and potential for success, rather than simply dismiss him as uncooperative.

"Can someone ask for a raise" of course.

Congralutions, you have just discovered that what is exciting to you is boring to someone else.

I sympathize with Grumpy. What would you have done if someone had brought you a project that was the complete opposite of what you liked. Maybe maintenance of a 40 year old Cobol based financial software. Would you jump at the prospect because your manager liked it ? It also involves some learning, somebody is excited about it and someone will pay well for it. Being open and clear about what one wants to do is not wrong.

The way I read this is he wants higher pay to compensate for the fact that he doesn't like the job you're offering. Most people would be willing to trade a better job for a worse job if the pay is high enough for it to be worth it to them, and he's just being frank with you regarding this instead of beating around the bush. I would take this at face value and not hold it against him.

Why is "extra responsibilities" in quotations as if it's strange someone would think added responsibilities would merit a raise? Isn't it generally true that extra responsibilities = raise? When an employee is promoted they generally get additional pay to compensate the additional responsibilities they take on. So of course they have the right to ask for the raise, they can always ask for a raise for whatever reason they feel. That doesn't mean you couldn't put them on the project without one, you just may not like the results.