Decrease of house allowances

Can my boss decrease my house allowances? I signed the contract almost 2 years ago.

The contract says they shall give me 250 BHD for house allowances but the situation at work changed and I have more duties and responsibilities now than before.

They didn't increase my salary or change my contract but we did a verbal agreement. We accorded they will pay 425 BHD for the house allowances.

Everything was fine, they did it for 1 year and a half but yesterday the new manager told me they are going to stop to do it because in the contract it says only 250 BHD. Is that legal?

After being doing it for a long time, can they change it like that? We have proof that the company is the one paying the house, also the house contract is in their name.

Thanks in advance :)