Ornamental fruit tree
I think this is glossy buckthorn, Rhamnus frangula. If you would upload a picture of the whole tree, and another of the bark on the trunk, that would help. Also take a couple fruits and cut them open so the seeds show- count the seeds in one and take a picture of all this.
Here are references for you.
If this is buckthorn it is invasive and should be removed. Thanks for using our service.