Second Birthday Invitation Verses How To Decline An Informal Invitation?

How to decline an informal invitation? - second birthday invitation verses

One recently had a new job. I quickly made friends w / one of my colleagues. Due to the nature of our work, we work together Saturday afternoon and evening. She asked me to join them today for her birthday. This includes dinner and drinks in a nice restaurant, not far from our work. I said, I go, would not work until then, I leave late (which happens sometimes)

Now I'm guessin second, whether I should go. I like the girl who is not the problem. But I do not want to go and spend money. I prefer to go my Fi and a cinema.

What should I do? I have just started to insult in this profession, I'm not at my lovely new colleagues.

It is not until later in the evening. They decided to open a restaurant, martini bar is a nice walk after 10 clock.