Background: I'm a software engineer in my mid-20s working at a small (~20 people) software/hardware company in southern Germany. I have ~5 years of experience, all at this company.
In brief, there are two key aspects to my question: 1) I want to move company because the pay here is too low for me, and 2) at the same time my bosses intend to promote me to a managerial or leadership position at the first opportunity. I will expand on these below.
1) Moving company: My salary is 60-65k EUR. I have aspirations to work at a tech giant or a financial firm in order to earn more and satisfy my ambitions. I am in a 5 month plan to prepare for those interviews. I especially want to leave because, after working my butt off for the past year, going the extra mile every time, and having an impressive list of the ways I've gone above and beyond, my boss declined to give me any raise at all. The reason was that I'm already being payed significantly more than my peers. That said, I already made plans to leave before this refusal.
2) Promotion: I have been displaying a strong ambition to be in a leadership role over the last years. Recently, my boss has revealed to me and reiterated that he expects opportunities to come up soon where he intends to make me the lead developer or project manager.
The fact that I'm torn between the career value of a promotion to manager but with lower pay versus being an engineer but with higher pay, is not my question (I'll ask that on a more appropriate forum).
My question is twofold:
A) If I choose to leave the company in a few months, what's a diplomatic way to decline the promotion, thereby preserving a great reference for my next employer? I have been pushing for this promotion for a while, so to the boss it will be disappointing and might disturb his plans.
B) If I am undecided, to leave or not, would it be terrible if I accept the promotion, and 3 months later look for the other job I have been preparing for? I could be undecided because maybe my market value is not as high as I thought; maybe I fail to join the companies I want to. In that case it would be great that I accepted the promotion and carry on with this career path. Otherwise, is there a minimum number of months that is acceptable to do this?