Albert Barnes on Romans 15:28

Have sealed to them. That is, have secured it to them. To seal an instrument of writing, a contract, deed, etc., is to authenticate it, to make it sure.. In this sense it is used here. Paul was going himself to see that it was placed securely in their hands.

This fruit. This result of the liberality of the Gentile churches-- the fruit which their benevolence had produced.

I will come, etc. This was Paul's purpose; but it is not clear that he ever accomplished it. Rom 15:24.

By you. Taking Rome in my way.

(k) "I will come by you into Spain" Php 4:17.