seal the bargain
1. To solidify, finalize, or decide upon an agreement or the terms thereof. I wasn't convinced at first, but it sealed the bargain when he offered to include a 10-year warranty for free. We both benefit from this arrangement, so let's quit stalling and seal the bargain!
2. To do something that marks or celebrates such an agreement. Everything sounds good. Let's seal the bargain with a toast! We decided to seal the bargain by opening up the 30-year-old whiskey I'd been saving up.
seal a bargain
and seal the bargainFig. to signify or celebrate the reaching of an agreement or bargain. They signed the papers and sealed the bargain by drinking champagne.