Fiscal year plan (1/31/24) overcontributes before end of plan year.

Client with calendar year 401(k) plan and fiscal year C Corp(1/31). The client (one man plan) made a 45k in December of 2023 which the accountant is deducting for fiscal year end 1/31/2024 and then a $30,500 contribution at the beginning of January of 2024.  The accountant says it was his intention to process a payroll in January of 2024 which he never did.  I am inclined to call it commingling of corporate assets with plan assets  from the beginning of January till February 1. 2024 and deduct it in the fiscal year ending 1/31/2025 calling it a prepaid contribution.  They just processed the clients 401(k) deferral for 2024.  It is my understanding you can not prepay a 401(k) deferral.  Does anyone have another idea about how to handle this contribution?  For the record I never tell someone to pay their profit sharing contributions before the end of the year... it just gives up options.