A sales commission is the property of the salesperson at the time that the commission is fully earned. Harris v. Investor's Business Daily, Inc., (2006) 138 Cal.App.4th 28 at 41, 41 Cal.Rptr.3d 108 at 118.
The issue for your daughter is when, under her employment contract, is her commission fully earned? This is a question of contract law interpretation. Commission agreements may be expressed in an employer's offer letter, a job description, an express contract/agreement, or implied by employer's prior conduct with the employee -- thus, the issue is not easily determined. With a very large commission, as your facts suggest may be in play, the issue may be worth litigating.
Assuming that the commission was fully earned at the time the new manager inserted himself into the account, then your daughter can recover the entire commission in court.
Hope this helps.
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