I just looked at a more scholarly text (published by the United Bible Societies) and it has instead of aphiemen the form aphEkamen.
The former is the present tense and the latter the aorist tense of the same verb. The aorist indicates an action took place in unspecified past time, without implying continuance or repetition--compared to the imperfect (the common past tense in Latin), the aorist is like a snapshot compared to a movie.
The alternatives aphiomen and aphiemen are found--the latter is apparently the form widely found in Byzantine manuscripts but is also found in the Didache, a 2nd-century Christian text, and in Clement of Alexandria (ca. 200).
The difference is between "we forgave" (aphEkamen) and "we forgive" (aphiemen). The Latin dimittimus is also present tense.