I work in the USA. During the Christmas time, the manager distributed Christmas cards to all his employees including myself. When he gave me the card wishing me Marry Christmas, I told him thank you, and later destroyed the card. Did I commit a sin by telling him thank you, even though I destroyed the card?

All praise be to Allah, and may his peace and blessings be on the last and best prophet and messenger, Muhammad.

Accepting the gifts of the non-Muslims in general is permissible due to the various reports indicating that the Prophet (blessings and peace be upon him) accepted gifts from Caesar, Cyrus, and other kings as reported by at-Tirmidhi and others from Ali ibn Abi Talib (may Allah be pleased with him).

Imam Ibn Taymeah (may Allah bestow mercy on him) reported from Ali that he accepted a gift from non-Muslims on the day of Norooz, which is a feast celebrated by many ancient nations, including the Persians, Egyptians, Kurds and others.

`Aisha was also once asked about accepting a gift from the Zoroastrians on the day of their feast, and she allowed it, given it is vegetarian (since their meat is not permissible to us). This last report was narrated by Ibn Abi Shaibah in his Musannaf.

Therefore, accepting the gifts of non-Muslims on the day of their feast is permissible, and that is the opinion of Imam Ibn Taymeah, who said in "Iqtida` as-Siraat al-Mustaqeem" that their feast will have no impact on the general ruling of accepting their gifts.      

Based on this introduction, it seems that you have done nothing wrong for it is expected when taking a gift to thank the person giving it.

Allah knows best.