[go] back on a (promise|deal|pledge)

to fail to keep or fulfill a commitment or assurance made to someone

She went back on her promise to help with the charity event.
The politician went back on promises made during the campaign, disappointing voters.
We had a verbal agreement, but she unexpectedly went back on the deal, leaving us in a difficult situation.
After months of negotiations, they agreed on a fair price, but he decided to go back on the deal at the last minute.
In marriage vows, couples promise to support each other, but going back on one's pledge can strain the relationship.