Temporarily removing fuel and replacing it into the same aircraft

Does anyone know of a FAR that prohibits removing fuel, avgas in this
case, and then putting it back into the same plane? The plane is flown
for part 91 service only.

Examples:

1.) The plane is 50 pounds overweight for conditions. 10 gallons is
siphoned out into a clean, single-purpose, approved for gasoline
container. The sealed container is stored in a safe place. 2-3 hours
later, upon completing the trip and returning to the field, I pour it
back into the tank.

2.) I want to calibrate a fuel dipstick, so I remove a gallon at a time
while taking readings. After I read the tank, I return the fuel to the
tank.

I've been told this is a violation. The person who tells me this can't
cite a FAR, and I can't find one in my own search.