Shortly after that scene, Marta takes Aaron to a flophouse and infects him with the virus, to 'viral him out' of the dependance on blue pills. He gets very sick. She is nursing him, wiping the sweat off his soaked brow. He struggles to sit up, even though she tells him to stay down. Aaron sits up anyway.
He then tells Marta that he has $40,000 in the lining of his jacket, there are passports in his backpack, both the ones they used and blank, and that the watch from the foreman is worth (I could not hear if he says $2k or $20k) and that she should take it all and go disappear. She has done enough for him.
I believe he took the watch because when he was talking to the foreman as the two of them entered the factory earlier, he noticed that the watch was of high value. He knew that their resources were limited and they could use an extra buffer. As they were attempting to escape the factory, the foreman ended up just in front of the pair, so Aaron took advantage of (either knocking the foreman out, or killing him) to aid their escape and since he was down anyway, Aaron takes the watch knowing its value will be needed in the near future.