We agreed a number before we knew the full effect

A number agreed in the corridor keeps the job moving, and that is its whole virtue. The vice shows up later: the figure assumed the straightforward version of the work, the complications arrived after, and the agreement - made to be helpful - now stands between you and the actual cost. Whether anything can be reopened depends on what exactly was agreed and how; often the honest answer is not much.

The lesson worth institutionalising is about the next one: price before agreeing wherever the job allows; and where a number is needed today, agree it expressly on stated assumptions - scope, conditions, quantities - so that when the assumptions fail, the price moves with them. That is drafting a pricing position, not a trick, and it is the difference between early numbers that help and early numbers that trap.