Negotiating aphorism, which urges a negotiator to ensure that one concession is definitely traded for another, rather than the possibility of another. Certain negotiating cultures are expert at appearing to present concession (a) to trade with a counterparty for concession (b), but obtaining (b) without actually surrendering (a). As a result, concession (a) can then be used again to trade for further concessions (c), (d) etc.
Alternately, where renegotiation of an agreement is sought, there may be a risk of simply re-buying the “horse.” Tactically this is often achieved by salami slicing or deploying the “blame God” strategy.