Albert Barnes on 2 Corinthians 9:3

Yet have I sent the brethren. The brethren referred to in 2Cor 8:18,22,23.

Lest our boasting of you. That you were disposed to contribute, and that you were already prepared, and that the contribution was ready.

Should be in vain. Lest anything should have occurred to prevent the collection. I have sent them that they may facilitate it, and that it may be secure and certain.

In this behalf. In this respect. That is, lest our boasting of you, in regard to your readiness to contribute to relieve the wants of others, should be found to have been ill-grounded.