If I gave more than 10% charity in a previous month, may I apply the surplus towards future months?
Yes, you can decide yourself the period on which you count tsedaka. Typical approaches are from Rosh Hashana to Rosh Hashana, or from January to December (so you know how much taxes you will pay).
Even from year to year you can use "credits" from over donating in previous years. Remember most consider tsedaka a very worthwhile minhag, not a strict halacha (see beginning of here)
Both R Avrohom Chaim Feuer in The tsedakka treasury and R Shimon Taub's in The laws of tsedakah and maaser) allow yearly accounting of once's maaser, i.e., you do not need to give maaser as income arrives as long as you "clear the account" regularly. This is also the allowance of The Chofetz Chaim (in Ahavas Chesed 18:2).
As I wrote here, the Chofetz Chaim offers the following guidance on setting up a maaser system (with personal additions to make it practical to the 21st century)
- Set up a separate ledger (or a spreadsheet) to track earnings, any business expenses, taxes and donations
- Designate a specific date (e.g., Rosh Hashana) to mark the end of an accounting period where you "close the books" - should be at least once a year - can be more frequent
- At the time of the closing period, calculate profits, deduct losses and acceptable expenses and the amount of maaser
- Deduct all donations already made and distribute immediately whatever you owe (i.e., the difference between maaser and previous donations)
- If you have given more than your planned maaser, some authorities allow to carry over to the next accounting period. Even those that are strict allow it if you stipulate (orally) that you may deduct from your maaser obligations at all times any donations you make at any time you desire
You confuse Maaser and Zedaka (charity):
Zedaka is practically unlimited and not refundable. You can give any amount to anyone anytime.
Maaser is a sort of "10% tax" on your net income. Opinions differ, but most hold that once you knowingly give more than 10%, the surplus is counted as a Zedaka, not as a future Maaser, as one can not pay taxes for future income.