I joined the company April 2018 as a Sales Executive. Now April 2019, my Boss is "promoting" me to a Managerial role (without pay increase, bad finances). This is not managerial training, or promotion prep. I have been "awarded" this new title (without the chance to discuss/decline), along with all the responsibilities that come with it, but they've not given me any contract or anything to sign.
My GM managed to negotiate a pay increment for me. BUT. I need to pass another round of probation (3 - 6 months) before I can get the increment. My Boss has not given an estimate of what my increment may be, but apparently it's "a lot of money - more money than I can get if I change jobs."
Thing is, my Boss is not the best at keeping promises. So, my question is: has anyone ever experienced something like this? Is this common practice in Singapore to go through another round of probation for a promotion? What can I do to protect myself?

I've never been promoted before, so I have no idea what the standard practice is. Any advice is much appreciated. Thanks!