The best is to ask him to sign the new resignation letter with effect from 31 December 2014. This is in line with the fact. However, please leave the date of the resignation letter blank for now – we will advise you later what date should fill in.
Another alternative is to call a shareholders’ meeting to approve the change of the company’s director. The process takes longer since there be 7 days’ notice period required, but it is also the efficient way to deal with this, irrespective of the resignation letter.
Please let me know which one you prefer.