Sell somebody down the river - Idioms by The Free Dictionary
sell somebody down the river
sell (one) down the river
To betray one for a personal benefit. An allusion to the practice of selling slaves to plantations farther south via the Mississippi River. These companies all want to pretend like they're your friend, but they'll sell you down the river the moment it makes financial sense for them. Part of the discontentment among voters comes from the sentiment that those in government sold them down the river when the economy collapsed.
sell somebody down the ˈriver (informal) act very unfairly to somebody who trusts you; betray somebody you have promised to help: The workers thought that their own leaders had sold them down the river.This idiom comes from the days of slavery in the US. A slave who was sold to an owner further down the Mississippi river would experience worse conditions than in the states further north.