Follow what is laid down in your probation terms and conditions and seek exit.
Go and meet the HR if they are co located or speak to them on phone.
What is the reason you are feeling they may not accept resignation?
Speak to them and I am sure they will give relieve you.Do not anticipate problems before they arise.
I think that the HR acts beyond his jurisdiction. Either he has to accept the resignation tendered with the offer to buy-out the notice period or reject the offer for reasons to be recorded and communicated.