While trying to turn a B757-200 for dispatch; a Mechanic robs a Public Address (PA) amplifier from another B757 at the hangar; but fails to have a write up made by himself or his Lead for the borrowed part.

Narrative

Aircraft X; arriving with inoperative PA; had history and all R&Es were busy. Although I am not experienced I took it upon myself to repair the problem in order to get aircraft out safe and on time. When aircraft arrived at gate; I cycled the PA amp CB with no help. Due to my inexperience; the repair process was slower than if an experienced R&E did the repair. I asked my Lead to order a PA amplifier to replace but was out of stock. We decided to rob one from Aircraft Y which was at the hangar. My Lead called the hangar and got permission to rob the part. I went to the hangar and removed the part from the aircraft. I assumed my Lead made a write-up for the robbed part; but I never followed up. I take full responsibility for this non action. My Lead was very busy and I was trying to turn the aircraft for dispatch. I installed the part on Aircraft X and was successful in the repair. I completed the paperwork for Aircraft X for dispatch. I was in a hurry and had problems obtaining the company part numbers due to only half of a company part number label missing and the small print on the other labels.