I am writing an academic article but I got confused as I am writing about the weight a specific metal got by using balance scale. Should I use the term mass, which has a unit of kg, or weight, which is a force created by gravity and expressed in N?
Because of the reasoning provided by you, i.e. mass is universal, and weight is the product of mass and magnitude of the local gravitational acceleration, I suggest to stay at mass. (A purist's view, taking @Ivan's comment into account.)
In addition, referring to the International System of Units, their brochure not only mentions the basic units, but the relevant section 18.104.22.168 starts with:
Unit of mass (kilogram)
It's up to you, ultimately. The Wikipedia entry for weighing scales states that:
The balance (also balance scale, beam balance and laboratory balance) was the first mass measuring instrument invented.
A balance or pair of scales using a balance beam compares masses by balancing the weight due to the mass of an object against the weight of a known mass or masses
the balance or pair of scales using a traditional balance beam to compare masses will read correctly for mass even if moved to a place with a different (non-zero) gravitational field strength (but would then not read correctly if calibrated in units of force).
In comparison to a spring balance:
Either type can be calibrated to read in units of force such as newtons, or in units of mass such as kilograms.
Technically, a balance compares weight rather than mass, but, in a given gravitational field (such as Earth's gravity), the weight of an object is proportional to its mass, so the standard "weights" used with balances are usually labeled in units of mass (g, kg, etc.).
Mass. Weight is mass in a gravity field or acceleration frame.