My boss recently found out about my second income as a semi-professional mid-range writer (not in English, so no worries haha), and now he's refusing to give me a raise at my full-time job. Currently, I have a contract with a publisher for a trilogy, with the first book set to hit shelves soon. I managed to negotiate an advance against sales of 5000 EUR and an additional 1k for granting them publishing rights to some of my previous e-books (which I had to take down from online stores). I haven't told anyone at work about my writing, as I prefer to keep it private. Although I'm not famous, writing is an intimate process for me, and I don't want to expose myself to people I don't care for.
Now, to the antiwork stuff. I made the most stupid mistake by leaving my contracts and paychecks on my office desk, and my boss found them and could not help himself but to read them. Surprisingly, he didn't mention it for a whole month until our individual evaluations today. During the previous evaluations, I was promised a raise if I met certain goals, and while it was only a verbal agreement, in previous years, such promises were always followed by a salary bump once I met the goals. However, this time my boss told me, “I really don't see the reason to give you a raise since you're already making so much money. You don't need more.” I pretended to be clueless and reminded him that he knows exactly how much I make, as he signs my pay slips. That's when he revealed that he knows about my writing and how much it has earned me. For context, my writing income is about half of what I make from my day job on a good year, including royalties (and before I deduct expenses like editors and marketing, which is mostly all of the writing income).
I tried to explain to my boss that my side income is completely unrelated to my performance at my day job, but he immediately referred to my contract, which states that “Senior employees are expected to devote their whole time, attention, and abilities to their employer's business.” And continued “Obviously, much of your time and attention goes to your writing, so don't even think about complaining, or you're close to breaching the contract and getting sued.” I'm struggling to understand how my hobby, which I pursue in my free time, is affecting my attention and time at my day job. Just because I get paid for my hobby, does it mean it's no longer a hobby? I'm not sure how to defend myself in this situation. I certainly don't want to breach my contract (and get sued and lose all the benefits like regulat income I can plan around, social and medical securities etc. plus I kind of do like engineering) , and on top of that, I believe I deserve a raise since I've raised my working standards and implemented desired changes in the workflow resulting in higher efficiency and customer satisfaction.
I would appreciate any advice or potential warnings in handling this situation. The problem is I am really worried about the lawsuit, and I have lost all my motivation for work since I basically cannot even do my hobbies because it might affect my performance. Lawyers are expensive, but is this a time perhaps for one?