Yes, in most county jails, the cash a person has on them at the time of booking gets processed and credited to their inmate account. It does not happen instantly, but the money does not disappear either.
When someone is booked, all personal property, including cash, gets inventoried and held by the facility. The cash portion is typically transferred to the inmate's commissary or trust account within a day or two of booking, sometimes faster depending on how the facility processes intake. Once it hits the account it is available for commissary purchases, phone calls, and other approved uses just like any other deposited funds.
The amount that gets credited is whatever was on them at the time of arrest minus any booking fees the facility charges, which vary by jurisdiction. Some counties deduct a processing fee upfront, others do not. It is worth knowing that going in so there are no surprises when the balance shows up lower than expected.
If the cash does not appear on the account within a few days of booking, the inmate should ask a staff member or submit a written inquiry to the facility's finance or trust office. Occasionally there are processing delays or administrative errors that need to be corrected, and a simple inquiry usually resolves it quickly.
In the meantime, if funds are needed urgently before the booking cash clears, family members can add money to the account through the facility's approved deposit method, which is typically an online platform or kiosk service.