Job offer letter

What US state?

Generally, in the US, absent a binding contract, your employer is allowed to change the terms of your employment.

 

You are free to quit anytime you desire.
The employer could terminate you anytime they desire without a reason.

Slavery was outlawed in this country during the last quarter of the 19th century.

The employer simply offered you a rate of pay.

The employer is free to modify the rate of pay.

You are free to decline and seek employment elsewhere.

The change might be undesirable, but its perfectly legal absent an employment contract to the contrary in FL.

Florida is an at will employment state.

Florida is an at-will state, meaning employers have the right to terminate a workers' employment for almost any reason – or for no reason at all.

 

All employment situations are contractual. However, most employment is on terms that are "at will", which means that both the employer (with some limitations) and employee may terminate the employment relationship at any time for any reason. Because of that, in an at will situation either employer or employee may at any time go to the other and propose a change to employment relationship and say "if you don't agree to what I want, this employment is done". Thus, at any time during an at will employment the employer is free to reduce your pay (as long as the reduction does not take you below minimum wage) and if you don't agree with the reduction, the employer can tell you to take a hike.

If you had a contract for a specific period of time (e.g. one year, five years, etc), that limited the reasons you could be let go, and that guaranteed your pay at a certain rate for that entire contract period then the employer would be unable to change your pay without breaching the contract assuming you had not breached the contract on your end.

Offer letters, even though they may be signed by both employer or employee, generally don't create a contract that has the sorts of things I mentioned in the last paragraph. They don't guarantee you a job for any period of time, they don't limit the right of the employer to fire you at any time, and they don't guarantee pay for any period of time. Most such letters I see say that the company will hire you starting some date for a particular rate of pay. All that gets you is that the employer will pay that rate for the work you do until the employer tells you the rate will be changed for future work.