I understood it like this:
[*]The client is on the construction site and is asked by a subcontractor whether he wants something done "this way or that way." [*]The client decides on a variant – knowing that this is not recorded in the contract and knowing that the subcontractor will immediately implement it. [*]The subcontractor carries out the request. [*]The developer invoices the service. [*]The client finds the invoice too high and backtracks, saying he should have received an offer and questioning whether everything was done correctly...
If it is as above, my assessment is as follows:
[*]The behavior of the subcontractor (thinking along and implementing as requested) is basically exemplary. [*]The behavior of the developer to charge for the additional work is of course justified. The price is naturally not cheap but also not unfair. [*]If the client now tries to somehow retreat to legal positions to revoke his on-site decision, I would find that shabby.