We don't have synonyms for cash boy yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are cashbox, cash box and call boy.
(n) :
(US, dated or historical, informal) In large retail stores, a male messenger who carried the money received by the salesman from customers to a cashier, and returned with the change.
(n) :
(US, dated or historical, informal) In large retail stores, a male messenger who carried the money received by the salesman from customers to a cashier, and returned with the change.