I used to do that all the time, back in the 1990s.
I worked for a company that made a small medical device, which packed up in a small suitcase-sized demo kit. The first time I took the sample in my first international trip was to London. I went through the "red line" at LHR and declared it as a sample. The Customs guys said that so long as I was taking it back out of the UK, there was nothing to worry about. They gave me no paperwork, changed me nothing and I went on my way.
From that day on, I never worried about declaring my sample kit in any country. I went to about a dozen countries, France, Germany, Portugal, Spain, New Zealand, Australia, Singapore, Thailand, South Africa, etc., and never once declared the demo kit. I never left it in the country, either. Never had a single problem, ever.
Those were my young and naive days.
It's possible that things have changed in the 20 years since I was in that job. My best advice to you would be to do as I did the first time. Figure on going through the "something to declare" red line.
Take paperwork with you from your employer stating that your sample kit is a Sample Kit, that it will go back home with you and not be sold and make sure that nothing in the sample kit looks particularly brand new. I took care to remove any and all plastic wrapping, cable ties, etc., that are usually discarded when someone buys a new product. Furthermore, I never traveled with completely-full bottles of calibrant; in fact for travel I avoided our standard liter-bottles and opted for small, travel-sized bottles labeled with a Sharpie pen. It saved weight and lent credence to this being a salesperson's sample kit.