Renovation: What to do if the quote differs from the final invoice?

Questioner

Due to water damage, it was discovered that the sewerage was outdated and had to be replaced. Various parties were consulted and ultimately party X was chosen. They gave a much lower price than the other parties, mind you, this was after we indicated that the old sewer pipes did not have to be cut out, but the new pipes could be laid next to them if that would save on costs. A quotation was presented on a time and material basis, below which is stated man-hours: €48,- / The work is carried out by 2 people and the estimated costs are between €3,500 and €4,000. It has been repeatedly asked whether 4000 euros is the maximum amount that the work would cost in total. This has been confirmed 3 times (verbally). Now the invoice is €7500,- and the work is not yet completely finished (there are still 2 skirting boards missing and leveling of the front garden after breaking open for the sewer). I have read something about a 10% legal increase for a guideline price. Does that also apply to a quote based on time and materials that gives a guideline price? What is my next best step? Request a specification of the invoice? Hold me liable for any damage that I have to bear myself to get the skirting boards and the garden in order? Am I legally obliged to pay 7500 euros? Or do I have to pay the 4000 euros that was initially agreed? I'd like to hear it.

Lawyer

A target price has been given and the contractor may deviate from this by 10%. The price must also be reasonable, in proportion and in line with the market.