The idea of the envelope budget system is that if you run out of cash in one category, you should stop spending until you've replenished it.
Of course, you can also transfer money from another envelope.
However, try to avoid this. This budgeting method is like a diet—it takes discipline. You're one slice away from eating the whole cake.
If you realize you didn’t budget enough for that category, set aside more money for it next month.