There is another problem no has brought up. Workman’s comp. If you get hurt doing the contract work you may not be covered under your employeers workman comp insurance, you are a contractor on an employee doing the side job. And If you do not carry your own comp insurance your employeer can be held liable.
As an example. approx 15 years ago while working for a department store chain this very thing happened. The company wanted a dock light installed for night drop off og trailers, this was not a maintenance issue and the department head refused to use his budget. The bid price came in around $1800 per store, 21 stores. Some of the engineers put in an offer of $1500 per store to be done after hours. The company agreed. One of the guys got hurt. State disability refused to pay because it happened at work. The employeer’s insurance co refused to pay at first, but bu agreement they did and then billed the employeer. Form that date forward there was no side jobs.