@Critter#2 those things are true but are more complicated to visualize and depend on the employer.
My employer will allow me to set an annual contribution amount and change it at any time. They look at what was already contributed, and divide the leftover by the number of pay periods. This employer may not do that. If he elected $100 per pay period for example, he may be stuck with that. That would mean he can contribute a lump sum of $4350 without getting into trouble.
Also, it depends on how much money he actually has on hand. Let's assume the taxpayer has $6000 in the bank to pay medical bills. Yes, he can pay out the $6000 to the doctors now, increase his payroll deductions to max out by the end of the year, and withdraw the $6000 on December 30 to pay himself back. But, the combination of paying $6000 out of pocket plus losing $500 per month in take home might be more than they can handle. Whereas contributing the lump sum on Monday and withdrawing it to pay the bills on Wednesday does not change his take home pay. While increasing the payroll deduction and paying the bills late may incur interest and late fees.
Making a direct lump sum deposit if the cash is on hand, and taking it out the next day, is legal, easy to comprehend, and not dangerous at all as long as the taxpayer does not go over their overall annual limit.