Wiktionary
vb. (context idiomatic English) to come to an agreement, to finalise the deal.
Wikipedia
Seal the Deal is the second full-length album by the Chicago based hard rock band The Last Vegas, released on April 10, 2006.
It was the band's second and last release on Get Hip Records before the split from the label. It was also the band's last release with bassist Anthony Rubino.
The album's 7th track, Raw Dog, was featured as a bonus track for the rhythm video game Guitar Hero II.
Usage examples of "seal the deal".
To seal the deal, Victor would marry Shannon O'Connor and Kieran would take Pauline Remillard.
Of course, Gillette had no idea of that when he came to me to seal the deal.
She put out a hand to seal the deal, as she did with her human clients, and laughed at herself as the cat sniffed her fingers.