Brother can't make you buy toner from them in order to preserve your warranty.
"The Magnuson-Moss Warranty Act of 1975 The Act does not allow tie-in sales. For instance, a printer manufacturer can't require you to buy an item, such as ink, or service from it to keep your warranty intact. The Act, however, does not require that manufacturers fix problems caused by third-party goods or services."
Not a lawyer, advice is worth the price paid.
From here:
https://yourbusiness.azcentral.com/c...nty-25320.html