Anyone have any ideas?

Here is a fruit tree of some sort in my back yard. The fruits have a 'pit' inside them that are similiar to a plum pit. It is hard to tell from the photo but they are the size of really big grapes but smaller than eggs. They are green but some at the top of the tree are turning pinkish. I have not eaten any of these but someone from work (an annonymous donor) dropped off something either the same or very similiar ...I tried those...sweet on the outside and very sour near the pit. They were pretty good.