That is an oversimplification. The commission is “earned” when the employer’s policy says it is earned. The employer’s policy may require the employee to remain with the company for a period certain beyond a sale for the commission to be earned. However, that would be addressed in the handbook.
IANAL. Realising that you may want to maintain anonymity, are you able to say what industry you’re in? The legality is going to depend on the specifics of your contract, and contractual practices are often consistent within a given industry.
My two cents is that if your job is straight commission, the company will be obliged to pay you, whatever you sign. It’s illegal for them to try to make you work for free. However, if you’re receiving a salary, and the commission is considered to be a bonus paid following the end of some period, then it most likely will be legal for the company to not pay out that bonus. Deferred bonuses are designed to allow companies to claw back owed payments if a sale or transaction is structured to be a short-term gain but a long-term loss. They also act as “golden handcuffs” to discourage successful employees from leaving.
I’ve run into this quite a bit. Some commission policies (e.g., for selling a service contract) state that the salesperson doesn’t earn the commission until 90 days (or some other period) after the sale. Otherwise, a salesperson who plans to quit could sell a lot of the product to friends and the homeless, collect the commissions, and leave the employer to deal with all the cancellations. I’ve seen this done more than once.
I don’t really have any advice for you, but I would probably share your gut reaction to refuse to sign. That could go badly for you, though; I have no idea.