As a PM I will need to listen to suggestions by cross functional stakeholders to work effectively.  The most important thing I would tell my Dev mgr is I will do my due diligence to research on the impact of the feature suggestion based on identify if there is a real customer problem that will be addressed, or a potential improvement that is not apparent currently.

I would prioritize then based on the impact and effort to be fair to all other features that are currently on the roadmap.

If the dev manager claims that it will improve stability due to addressing tech debt, I would work with him/her to identify upcoming sprints to balance the work.   Increased stability translates to higher uptime and performance for the product, and it can still move the needle.