Ufff! He is trying to act too smart. By the way, be careful with such guys because they always end up with some issues. That's their attitude.
Just send him an email, copying the HR Manager and his Reporting head, stating that "In reference to the offer and appointment letter, Mr. XXX has joined the services of XYZ company on Jan 10, 2011. In accordance with the company policies and the offer letter, the company is obliged to pay salary from Jan 10, 2011 only. Hence, we have credited the 22 days' salary to your account."
If he still comes back to you for 2 days' salary, then tell him that only his date of joining speaks. Even if he argues, ask him why did he accept and sign the letter dated his joining as Jan 10, 2011. If he had this kind of thinking in his mind, then he should have asked for Jan 8, 2011. So, no mistakes lie from the company end.
Still, if the issue persists, then bring all the possible people that you can gather (Top management people). Put this issue on the table. He will get good hits from them.
If he still argues, then my advice is to get rid of people with such behavior. They will spoil the office environment. Issue him a warning letter. Everyone should follow the policies of their company; instead, no one has the right to formulate policies of their own and go according to it.
Regards,
Babu