One Sided Manual Match possible?

We have configured Transaction Match and within a Match Type have configured a Match Process with one data source. We have the Manual Match rule set-up on the single data source match process to match on the Amount attribute. However, within the unmatched transactions screen, users cannot "match" on transactions that have a zero variance. For example, matching two or more on transactions in the same data source that net to 0. The "Match" button is greyed out. Is it possible to have manual one-sided matches? Am I missing something on the configuration side? Thanks!