Was offered higher salary "later", do I push for "sooner"?

Question

1. At company X, I was offered a job at "The new office", A, which "We will be opening soon".
2. The manager of the "new office" resigned, office A did not materialize.
3. While a replacement for the manager was sought, my contract got placed with office B, while I started work at office C, which is across a nearby country border.
4. Six months ago, everyone agreed that I would stay at C for now, and so my contract was transferred there.

At the time, I was only vaguely aware that salaries were higher across the border. Now my current boss, at C, has without previous inquiry from me on the subject sent me an e-mail saying, in short

You are entitled to a higher salary, in 6 months you'll get a 20% raise.

My question is, if I should have a higher salary, is there a reason I should wait 6 months to get it? Is there a reason I should not have it retro-actively? Am I just being greedy?

I read the FAQ, but I'm still uncertain as to whether this is on-topic or not. After reading this meta question are-questions-about-careers-off-topic, I decided to post.

Answer 1

The question is why are you entitled to a higher salary? '

If it is a contract or minimum wage consideration then you absolutely have the right to ask for an immediate raise. It is also possible that the contract/raise will take place in 6 months and that is when you will be entitled to it.

If you are not actually entitled to it but your company feels that the range for the position is between $X and $Y and your current salary is 20% below $X then you are not truly entitled to it.

I would approach it with my manager and ask him why I am entitled to this 20% raise. Ask him/her why it will take 6 months to get it. Those are answers you can then use to make your decision.