Jamieson, Fausset and Brown on 2 Corinthians 8:6

6. Insomuch that--As we saw the Macedonians' alacrity in giving, we could not but exhort Titus, that as we collected in Macedonia, so he in Corinth should complete the work of collecting which he had already begun there, lest ye, the wealthy people of Corinth, should be outdone in liberality by the poor Macedonians.

as he had begun--Greek, "previously begun," namely, the collection at Corinth, before the Macedonians began to contribute, during the visit to Corinth from which he had just returned.

finish in you the same grace--complete among you this act of grace or beneficence on your part.

also--as well as other things which he had to do among them [Alford].