GOING WITH DEREK TO A MEETING WITH THE WORKERS:
There's not too much to figure out here, because all you have to do is to use the stairs #1 and follow Derek and his men to one of the warehouses #2. The next cut-scene is very important, because Vito will find out that Derek was responsible for his father's death and will decide to go after him.
Start off by choosing a good weapon for long range combat, because the shotgun you've received from Derek won't do the job. Exit the warehouse, take cover behind the crates #1 and attack the first group of Derek's men. Keep heading forward, eliminating other gangsters along the way. You'll soon notice that a new group has arrived in the car #2 and it'll be up to you to get rid of the reinforcements.
You'll have to be careful, because some gangsters may separate from the main group and you'll have to prevent them from making surprise attacks #1. Naturally you should rely on your radar to find them. Keep heading towards the warehouse located next to Derek's office #2, because that's where he'll be hiding. The warehouse door will close behind you, forcing you to carefully plan this boss fight.
Ignore Derek for the time being and focus on defending your position against other gangsters. They will appear in a few seconds and you'll have to prevent them from moving too close #1. Derek will probably start throwing Molotov cocktails once they're gone #2, but avoiding the flames should be easy. Remain near the gate, so you won't take any unnecessary risks.
The next part of this boss fight will require you to eliminate four additional gangsters. They will use the stairs to get to you #1 and this is when they must die. You can't let them get to the ground floor, because they would spread out and eventually kill you. Focus on Derek after all of his bodyguard are already dead. You won't have to go to the upper balconies, because you may start shooting at him from the ground level. All you have to do is to wait for him to lean out and use a good weapon to kill him #2.
You may proceed to the upper balconies. Examine the area near Derek's body #1 and you'll find a lot of Molotov cocktails. Proceed to Derek's office and take almost 23000 dollars from the desk #2. Leave the building.
GETTING THE REST OF THE CASH:
If you didn't spend too much money during this chapter on car repairs and new clothes you'll probably need additional 1500-2000 dollars to gather 27500 dollars needed to pay off your share of the debt. There are two ways of getting the money you need.
A much safer way is to deliver vehicles to Mike Bruski's scrapyard located in the northern part of the town. Each time you've got a car you must drive it into a large car press #1 and then interact with a nearby control panel #2. Delivering a car should allow you to earn about 400 dollars. Sadly, you will have to leave the scrapyard in search for new cars so the entire process of acquiring about 2000 dollars may take some time.
An alternative solution is to start robbing the stores seen on Empire Bay map. It's a much more dangerous activity and each robbing has four phases. Start off by stopping the car in front of the store #1. Make sure to leave the door opened so that returning to the car won't take a lot of time. Enter the store, draw a gun and kill the owner #2.
Quickly approach the register and take the money #1. Once you have the cash return to your car parked in front of the store #2. If you manage to do this before the cops show up nothing bad should happen. If you're caught by the cops while stealing the cash or while trying to leave the store you'll have to escape them and that's why it's a good idea to use a fast vehicle for robbing missions (Joe's Smith Thunderbolt is an excellent choice).
If you're more interested in robbing than delivering cars to the scrapyard you should consider attacking gun stores #1, because each operation will result in earning about 500 dollars. Be careful, because owners of gun stores will be using shotguns to defend their stores #2 and you can die if you don't shoot them right away.
Continue earning cash until you have at least 27500 dollars in your pocket.