401(a) defined contribution plan: tax treatment of contributions made outside of payroll

I have a defined contribution plan (401a) through my work. I was on leave without pay in 2020 and in order to receive the employer matching I made direct payments to the plan through my benefits office. Because the payments were made outside of payroll they were post-tax. Is there a way to claim a tax deduction or to otherwise alleviate the double taxation that will otherwise result from my eventual disbursements in retirement?

 

Thanks!