A.T. Robertson Commentary Romans 15:28

"When therefore I have accomplished this, and have sealed to them this fruit, I will go on by you unto Spain." — Romans 15:28 (ASV)

Have sealed (σφραγισαμενος). First aorist middle participle (antecedent action, having sealed) of σφραγιζω, old verb from σφραγις, a seal (Romans 4:11), to stamp with a seal for security (Matthew 27:66) or for confirmation (2 Corinthians 1:22) and here in a metaphorical sense. Paul was keenly sensitive that this collection should be actually conveyed to Jerusalem free from all suspicion (2 Corinthians 8:18–23).

I will go on by you (απελευσομα δι' υμων). Future middle of απερχομα, to go off or on. Note three prepositions here (απ' from Rome, δι' by means of you or through you, εις unto Spain). He repeats the point of verse 24, his temporary stay in Rome with Spain as the objective. How little we know what is ahead of us and how grateful we should be for our ignorance on this point.