Timing Solo 401K Employer Contribution

My client has a Solo 401(K) and is asking if they can front load their employer match (25% of salary) since they know what the total contribution will be for the year. To make the numbers easy, let’s say the S Corp pays a $72,000 annual salary ($6,000 per month), and they want to make the maximum employer contribution for the year ($18,000). Does it matter at what point in the year that contribution is made? For instance, provided that the LLC does in fact end up paying the client $72,000 by year-end, would it be a problem if all $18,000 had been contributed by the end of September, even though the client had only been paid $54,000 by that point? My understanding is that the 25% limit is only checked at the end of the year.