In CA, it is true that they can (actually, must) get interest.
While most trust accounts are under the probate code (in CA), the Insurance Code requires trust accounts for certain independent Agents, the Real Estate code requires trust accounts for client deposits not put into escrow, and the Residential Care Facility code requires a trust account.
Insurance & R/E are pretty rare, and the Agent has to have "permission" to earn interest, but the Residential Care is different. It is a Trust Account established by law as opposed to a trust established by Trust Agreement. The facility cannot allow residents to have more than a nominal amount of cash to mitigate theft. The law requires strict accounting for the funds and the interest earned must be allocated to the residents based on their deposit balance.
Your customer should be able to provide the specific TX statute for you to review. The accounting is audited by the state licensing board.
Al
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Al Miller, CRCM
Opinions expressed are my own and not necessarily shared by my employer.