I'll give it a whirl...
Payroll = money obligated to currently signed players
Expenses = money spent during the season which often differs from payroll due to signings, releases, trades etc
Basically, your payroll fluctuates during the year depending on signings, trades, minors call-ups (when you call up a player with a minor league contract they automatically get a major league contract at 300K salary which is added to payroll) while the expenses is what you've actually spent during the year after accounting for all the moves made. Your expenses can be much greater than your current payroll at the end of a year and sometimes less depending on transactions made for the year.
When releasing a player with a year left on their contract, it depends on when you release them.
1) Release out right during the season or at seasons end prior to roll over to new season: You pay this years salary and next years salary all this year, so your expenses will be the existing payroll plus the additional year of salary for the player. For Ocampo that would mean that next years 3.9 million is added immediately to this years expenses.
2) If you wait and release the player at the start of next year: You pay the normal yearly salary for this year and then next year you pay the normal salary even when released before the start of the season (in the off season.) This way you don't have to account for the next year of salary this season saving your expenses and next season, since it's the players FA year, you wouldn't accumulate additional expenses above the normal payroll obligation.
Basically, unless you're trying to clear roster space immediately it's probably better to hold off and release the player during the next years off season period.
Of course, there are other factors to consider. Since you can only "save" 15 million in your cash fund from year to year then if you are going to end the season with a surplus that puts you over the 15 million limit then you may want to drop the player so you don't lose the money over 15 million.
Example:
Payroll 75,000,0000
Expenses 75,000,000
Income 80,000,000
Cash 13,000,000
Since you'd be making +5,000,000 (income - expenses) and already have 13,000,000 cash you'd lose 3,000,000 overall because only 2 mill of the 5 mill profit will get added to your cash. If you dropped a player with a year left making 3.9 mill before the season rolls over then you'd only lose 900K since the new numbers would be
Payroll 75,000,000
Expenses 78,900,000
Income 80,000,000
Cash 13,000,000
The income - expenses is now only 1.1 million so the 13,000,000 cash would become 14,100,000 and you'd not "lose" the additional 3 mill which you would have in not releasing the player.
Hope that helps somewhat.
Vienna Buzz