Which he obviously didn't always do and why he got hit with tax evasion charges.
What we learned from the Fani Willis / Nathan Wade case is that retainers to lawyers are put in an IOLTA account, essentially an escrow account where the client's money is held until the lawyer invoices the client for performed work. The lawyer is then permitted to transfer the invoiced amount from the IOLTA into their personal account as earned income.
We also learned from Nathan Wade that he routinely billed Fani Willis a monthly amount of $35,000 (his monthly cap), which was supported by a minimally detailed set of invoices.
The point is that the client, when writing the check for the retainer as "legal expenses," does not know at the time what those legal expenses will be; a retainer is for future work, not work previously performed.
-PJ