I can think of several issues:
1. Your broker wants cash to avoid reporting the fee as income and avoid paying taxes on it or reporting it to the brokerage firm and having to split the fee with another broker.
2. Did you sign a broker agreement and did your broker explain the fee to you and form of payment? Did your broker suddenly tell you to pay in cash after the agreement was signed?
In either case, you have the right to pay your broker with a bank or certified check or money order. Never pay in cash. If your broker refuses, you deserve a truthful explanation why you need to pay in cash. Explain to your broker that you have the authority to notify New York State Department of Licensing and your local Board of Realtors that your broker took an oath to obey the Code of Ethics and to uphold his/her fudicuiary responsibility. Professional licensed brokers don't accept cash as a form of payment unless its for a service outside a transaction. All broker's fees to close a transaction are paid by bank or certified check.
I am a retired real estate salesperson.