Unless you applied or received notification, you have no contract with the IRS, and the IRS has no statutory jurisdiction to intervene in the private affairs of you and your "employer" as they relate to the voluntary withholding agreement (a.k.a. W4). If one want to pursue the matter further, the following question could be posed to the employer: "Are you the Secretary's designated agent under 26 USC section 3504? Please show me a copy of Form 2678 if you are ".. 1545-0748 is the OMB control number for 2678. This form is necessary for some people (employers), and they are listed in ADP IDRS Document 6209 Employment codes 3-16. Your employer is very certainly not mentioned there; if he is, you're out of luck.
You should also point your employer to 31 CFR Part 215 and 27 CFR Part 70.161 et seq., specifically at (a)(4) (A)(B)(ii) and (a)(4) (A)(B)(ii) and (a)(4) (A)(B)(ii) and (a)(4) (A)(B)(ii) and (a)(4) (A)(B)( (iii). There can be no liability issue relating to 26 USC as far as your person is concerned unless you receive a notice from the Secretary, as required by 26 CFR 1.6001(d). See, for example, 26 CFR 301.7512-1 b, c, and d, and again, ask your employer if he is following the rules; after all, he claims to be handling the government's money, and that responsibility should not be treated lightly. The government's agents are often hostile to any deviation from the plan.
Finally, you can go to pages 2-1, 2 and 7 of ADP IDRS Document 6209 Tax Returns and Forms, and then ask your employer if he expects you to make this donation in accordance with 31 USC sec. 321. I doubt your "human resource manager" (anyone else insulted by that term?) will bring this information to his "attorney," and if he did, I'd bet three to one that even on his finest day, his attorney wouldn't be able to explain it to you.