Income
Rule: Don't buy into a round you can't afford.
There are of course exceptions to this. If you are out with a small group of familiar fellow drinkers and your lack of liquidity is temporary, your fellow drinkers may be happy to donate funds to you. If this comes in the form of cash and you are to act as though you are in the round, then that money is a loan and must be paid back. If someone else simply buys a round for you, then you don't owe that person money, but you do owe them the courtesy of buying them a round when your cash flow returns to a more healthy state.
Drinking with the boss
Lisa, Eve, Mark, Matthew and Luke are out at a bar for after-work drinks. Luke is the boss, and he initiated the after-work drinking session. The income variation is quite large and Luke earns considerably more than the others. Eve and Matthew earn the least, and generally don't go out. But they felt obliged to join, not wanting to be left out. The question is - should Luke be buying all the drinks? The answer is - No. Employer-employee relationships are irrelevant with regards to round etiquette. You are all equals in the eyes of the Round God.
Luke (the boss) may wish to start out by buying the first three or four rounds, but this is a matter of choice, and should not be expected.