Assistance with legal questions about contracting

Questioner

A contractor has made a verbal agreement with a homeowner about a renovation for €40k based on mutual trust. During the renovation there are setbacks and additional work that were discussed verbally but not agreed upon. The contractor invoices the contractor in installments without substantiation with an invoice for additional work for €15k on top. All these invoices (total €55k) are paid by the homeowner within the payment terms. During construction the contractor sends quotes for another €15k that are not accepted by the homeowner with the explanation that all work performed would be paid for well within the €55k already paid. Three months after the work is finished and the invoices are paid the contractor verbally claims €15k in outstanding payment (without invoice). Question: What should the homeowner do now to formally close the case and not run the risk of receiving an invoice and reminder later?

Lawyer

The contract for the acceptance of work can be against an agreed (verbal) fixed price or against a target price (also in hours). If a fixed price has been agreed including material, then that is the price. I assume that a target price has been agreed, from which a contractor may legally deviate only 10%. For all additional work and more material, it applies that it has been informed in advance that this increases the price, otherwise the fixed or target price applies (plus possibly that 10%).