But that seems not unreasonable on its own. Tipping a friend $20 because he helped carrying boxes when you moved as compared to hiring a professional mover for $20 seems like a valid difference to me. But you a right that this could maybe be turned into a good argument for an universal basic income because you may be able to sidestep some otherwise relatively difficult to solve gray area issues, but I am not sure, I haven't thought that through.
All of which perpetuates the cash/grey economy (and the off-shoring of garment-making jobs in your example).
The thing about UBI is that it allows employees to turn down bad economic deals without starving. Sure, you can't live in a 2000 ft^2 house, drive a new car, eat and party out all the time, but you probably don't have to take a job with ridiculously bad conditions just so you can buy a can of beans or Ramen.