I've lost 30% of my salary 😱 - Any idea how to make some extra money as a Dev?

Start a side hustle. This is why multiple income streams are so crucial. Check out https://indiestack.org/ to build yours.

Frankly, if you have time 'till October, the chance that you build any meaningful income stream, is quite slim. You could try UpWork, but even there, my experience as an employer is that companies typically look for full-time programmers.

I would suggest something else: start looking for a new job already. So that in October you can get the highest salary possible - to compensate for the money you're losing now. Take the time to get several offers as that would give you better negotiating power. Personally, this is what I would do; if I knew I need a new job in October or November, I would be already talking to recruiters now.

Alternatively, you can spend the time learning some new tech that would increase your value on the market in October. You have ample time to learn a new language or two, or some popular frameworks.

Hope this is not too discouraging.

Unfortunately, as great a career as programming can be, it's still a job, with labor exchanged for money. Without something to sell and putting in the work to get people to buy that something, the only real option is "take more jobs," which limits your time for other things, like finding a better job, and increases the risk of burning out. There are many marketplaces (like CodeCanyon, as you mention, plus the various app stores) where things can be sold, of course, but without a sales push, a developer is really just hoping that people stumble on it, so it's not reliable money.

If your employer isn't paying you your agreed-upon salary, though, that might (not legal advice) be a legal issue (wage theft is a big deal, though generally not in salaried positions) that could get resolved faster than building a customer pipeline...

You can use Upwork and find some freelance job there

Do Programming Custom work eg via upwork

Thanks for all the suggestions here, I would use this advice

I'd recommend looking into freelance to fill in the gaps. It'll take you some work to land your first few gigs, but freelance is nice because you can control your workflow. I'd recommend getting your portfolio website up if it isn't already and checking out UpWork and Fiverr for gigs.

+1 to the suggestions you create a side hustle. You can go into freelancing or create and promote a digital product. Ask friends and peers if their companies need someone with your type of expertise and let them know you're open to taking on short-term projects.