Am I obligated to offer expiation for an oath, and what should I do, if I said to a female colleague: "May Allah neither absolve me nor permit me if I give you anything from my belongings," and then I subsequently gave her some of my belongings?

The wording mentioned is not an oath by Allah, nor by His names, nor by His attributes, and therefore it is not binding. Although some scholars hold the view that an oath is binding by mentioning Allah's name without a particle of oath, the context here is supplication (du'a). Al-Nawawi stated that mentioning Allah's name without a particle of oath does not constitute an oath unless intended as such, according to Al-Shafi'i, whereas the Hanbalis believe it does constitute an oath. If the intention was to suspend the prohibition of something permissible upon a certain matter, then prohibiting what is permissible is null and void, and nothing results from it. Therefore, the questioner is not obligated to do anything regarding giving some of her belongings to her colleague.