My ex bought me a pro camera a few years ago (when we were a couple). It cost around 1000 euros at that time. He said that for it I need to make 14 days of photoshooting for him. Since we spent a lot of time together, I made a lot of pictures for him and did not count the days. Now we are not together and he asks me to give the camera back. Our agreement was not written, it was verbal. Can he sue me saying that I did not follow the agreement?
There is no law against your ex asking for the camera back (up to a point- this could be considered harrassment if it went too far).
I'll say the obligatory line here- anyone can sue anyone for anything- but it might not go anywhere. Since you don't want to give the camera back, his next step would be to file in a small claims court or similar (depending on country, though I believe most European countries have something similar) for the camera or its value.
From what you're said, this could be considered a verbal contract. In that case, you could be expected to complete the contract or return the camera. It's worth pointing out that if the contract is voided, you would likely be able to keep the photos taken.
If, however, the court found that either the camera was a gift or that the contract had been fulfilled, it would legally be yours.