Honestly I feel like the boss trying to lowball you.
I know downstream oil palm is all the rage now, and even mills in Indonesia looking to hire from Malaysia with generous package. So for them to offer a measly RM300 above your current pay is quite underwhelming.

For me I have to agree that career development wise, it's good to be onsite at the mill. Only downside I feel they're ripping you off with the current offer. So either :

i) Negotiate for better package. Ask for all expense paid, suffering allowance etc.
ii) If they force you and make it no choice, go there, gain the experience, and actively look for vacancy with competitors.