Estimation and negotiation challenge

Hi All

I hope you can help me

I am software engineer, I have multiple years of experience in various roles

I have done estimations but they were for internal stake holders and mostly in "agile" way

Now I am facing challenge to provide estimations for client (external stakeholder ) and negotiate them for piece of work where scope of work is tricky to estimate

Current piece of work is to upgrade about 20+ dependencies for legacy monolith project, web portal.

The amount of work it is difficult to estimate, it can explode, and some issues can appear on live

It is fixed price project

I would appreciate if you could share your your experience in similar situations and thoughts especially concerning providing estimations and negotiating them

Thanks in advance