If I realized that I have been paying too much maaser each month due to not deducting certain qualifying expenses from income can I recoup the overpayment from future maaser?
The Noda Beyehuda [volume 1 siman 73] rules that one may give preemptive maaser on many he has not yet earned and deduct it from later earnings. This is quoted in Pischei Tshuva [Y:D 249]. This is also the opinion of the Chasam Sofer in his chiddushim to Maseches Gittin 31b. This would apply to your situation as well, and you may deduct from future earnings.