Before getting anything in writing, you should step back and ask yourself how you feel about working in a company in which your manager’s word does not mean anything unless it is accompanied by a signed document. If you cannot rely on what your manager says, that actually says a great deal.
Your manager may indeed react negatively the lack of trust associated with being asked to sign anything regarding a future promotion, and merely asking could damage your working relationship as well as the possibility of a promotion. In fact, your manager might not even be allowed to enter any formal agreement regarding your retention or promotion.
A more positive strategy is to send him an email thanking him for indicating that you are next in line for a promotion. This type of note can serve as a reminder as well as a way to document his comments. Be sure to save this email and keep a hard copy of it, and do the same if he happens to respond.
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