I was looking for a promotion. Not seeing any reaction from management, I started interviewing. Now I am about to get an external offer (I don't know yet the economics). But my company came back to me these days telling me they will give me some raise this year (again I am yet to know the ammount). 

What should I do? Tell both sides what's happening? Will they think I am gambling? What to do if both offers appear to be equivalent?

EDIT: more info

Things are going slow on both sides. I didn't told anything. I still don't have anything in writing. And the external company in the last interview (which went well again btw) told me they will first call me to explain their offer and see if I agree before putting into writing. I am on my current job for 10 years. I like the place and I don't have problems with the management team. I am more senior than almost all of my peers and as such I am appreciated in the company (but not economically). As stated the problem is the money. Sure sometimes I have to deal with lazy people doing nothing while on remote work and this annoys me, but I feel I will get this everywhere. Thank you for the comments!