Advice on Legal Concern

My manager was the client at the time and instructed work to go ahead. All planning, Principal contractors and designer were instructed to install a septic tank. Initially, it was declined by local authorities to connect to the main sewage system. However, when the project started, the hole was dug and was not strong enough to contain the tank, the hole was subsequently filled back in and is now after negotiation connected to the main sewage line. The company who were instructed said they would carry out the install to the main line at the same cost of what the septic was going to cost. The company has now invoiced my company and is asking for 15K more than previously quoted for labor/materials on top of the install price. My question is - is this feasible and allowed or is them trying to claim back costs even at the time it was already agreed?