Rate increase subsequently decreased

[carovna]

I have recently started a new job on an agreed rate p/h. A week after I started I received a letter stating a rate increase p/h which was backdated. Nice!
Two month since then I have received another letter stating the rate increase was incorrect and they are going back to the original rate - also backdated and they are going to deduct the whole amount from my next pay at once.

I think that's very unfair and I'm really upset about this. Where do I stand legally?

Please advise