John Chrysostom

"When therefore I have performed this, and have sealed unto them this fruit." That is, when I have laid it up as it were in the royal treasuries, as in a place secure from robbers and danger. And he does not say alms, but "fruit" again, to show that those who gave it were gainers by it. "I will come by you into Spain." He again mentions Spain to show his forwardness (aoknon) and warmth towards them.

Homily on Romans 30 ↗

Theophylact of Ohrid

That is, as if having deposited it in the royal treasuries, as an inaccessible and secure place; for such is that which is sealed. By saying "fruit," he shows by this that those who give alms receive great profit from it. Again he mentions Spain, showing thereby that his journey there is necessary for him and that he will pass through Rome not out of contempt for them.

Commentary on Romans ↗

Jerome

One poor man, already old, preached from Jerusalem all the way to Illyricum, and swiftly runs his word. Since, then, it had come from Jerusalem all the way to Illyricum, and his word had run swiftly, next it follows, and that very word which had said, From Jerusalem all the way to Illyricum, says: But I wish to go even to the Spains. This the apostle Paul speaks. See the swiftness of the word. He is not content with the east: he longs to run to the west.

Homily 57, on Psalm 147 ↗

Thomas Aquinas

Then when he says, when, therefore, he mentions the place from which he will come to them, saying: when, therefore, I shall have accomplished this, namely, ministering to the saints, and consigned to them this fruit, i.e., the alms of the gentiles, which is a fruit of their conversion: Israel is an abundant vine that yields its fruit (Hos 10:1); I shall come by you into Spain. But the Apostle seems to be saying something false here; for there is no record of his ever being in Spain. …

Commentary on Romans ↗