Fraud in construction acceptance - incomplete termination of construction measures

Construction of single-family house, water connection, drainage, infiltration... Our civil engineer concluded a lump sum contract with us. After not quite half of the work was completed, we paid two interim invoices totaling 45% of the lump sum amount. That was in November, then he continued the work from March to early April and then simply stopped unfinished. Since there are defects and he breached the contract, we had to terminate him in May. This was followed by a final invoice which we did not pay, let's call it billing fraud, theft, retrospectively invented "additional orders". He refuses acceptance because he allegedly accepted the work already in November. Later he wrote "through use" of the shell construction – we held the topping-out ceremony then – the house was only completed 8 months later and then also the move-in. Does he have to carry out an acceptance as demanded by me? The court accepted his lawsuit and did not consider our request for rejection due to lack of acceptance and even in the first hearing the acceptance before completion of the trade initially played no role. According to VOB the final invoice is due when acceptance has been made...

Legal advice only from a lawyer, and with [VOB] you will probably come off worse, because here the developer usually has the advantage. But it won't help; if you want to come out of it unscathed, you have to go to a construction law attorney.