Why doesn't Paul use the word tithe?
Paul does not use the word tithe because tithing was part of the Law, whereas the believer's giving is not Law-based. Yet that does not mean God does not expect giving. In the context, Paul speaks instead of a collection for the poor saints in Jerusalem.
Les explains that Paul doesn’t use the word tithe because that was part of the Law, but that doesn’t say God doesn’t expect you to give. Instead, Paul writes concerning the collection of money for the saints, remembering the poor saints in Jerusalem. Those saints had given everything they had in Acts 2 and 3 in light of the Kingdom, but the Kingdom didn’t come. God in His providence took care of them through the offerings of Paul’s converts from Gentiledom, brought back to Jerusalem. Paul ordered the churches of Galatia and Corinth to collect on the first day of the week, not the seventh-day Sabbath, because it was Resurrection Day.