How to say job offer in Mandarin/Cantonese?
'Job offer' is a noun.  It means 工作提案 in literal sense
聘用提案 (hiring offer) world be a better Chinese counterpart
you can also use a more literary term '邀聘' [invite to a job(v); job invitation (n)]
Example:

大飯店開張要大量招聘各部門員工
The opening of a grand hotel requires hiring of a large number of employees in various departments.

(招聘 means calling for people to apply for the positions)

而行政總廚一職我想邀請XX酒店的大廚擔任
And I want to invite the head chef of XX Hotel to serve as the executive chef.

(邀請 means 'to invite')

但XX酒店的大廚對這個邀聘看法又如何？
But how the head chef of XX Hotel thinks of this  job invitation?

邀聘 as a verb means 'invite to a job' (邀請受聘)
邀聘 as a noun means 'hiring invitation' (聘用邀請)
(This job is only offered to you, we are not opening this position to the public)

最後XX酒店的大廚接受了我們的邀聘
Finally, the head chef of XX Hotel accepted our job invitation
最後XX酒店的大廚接受了我們的聘用提案
Finally, the head chef of XX Hotel accepted our job offer

side note:
In Cantonese, 'hotel' is '酒店' , 'head chef' is 大廚 or 頭廚
In Mandarin, 'hotel' is '飯店' , head chef' is 總廚