That's not how it works in both Germany and Austria. If you have a contract you get paid based on it, if there is a bookkeeping error you have to pay the money back if the company accidentally gives you too much.

The only contracts that are invalid are when the number is very obviously wrong in the context. For example the contract says instead of $50k a year you get paid $500k a year or $5k a year, then the entire thing is void as it's an obvious error.

If the contract says $55k and the company wanted to pay you $45k.. their problem, contract counts. Your boss might be pissed if you keep insisting on the $55k and might fire you, especially if you verbally agreed on $45k. But oh well, that's another topic.

Oh and in the UK? The employer is even allowed to deduct that money from your future wages. So much about knowing the law :)