You have - as far as you have published texts - no leverage to withhold funds!
Rhenish greetings on the road
Bauexperte
Stop! That is WRONG!
A consumer who has concluded a contract for the construction of a house must receive a security (e.g., guarantee) amounting to 5% before the first installment payment (§632a Construction Code).
If this does not happen, you can deduct the 5% from the total construction sum from the first installment payment or (initially) do not have to pay at all.
(3) If the customer is a consumer and the contract is for the construction or conversion of a house or a comparable building, the customer must be provided with a security for the timely completion of the work without significant defects in the amount of 5 percent of the remuneration claim at the time of the first installment payment. If the remuneration claim increases by more than 10 percent due to changes or additions to the contract, the customer must be provided with an additional security amounting to 5 percent of the additional remuneration claim at the next installment payment. At the contractor's request, the security is to be provided by withholding the installments in such a way that the customer withholds the installment payments up to the total amount of the security owed.
According to the Federal Court of Justice (BGH) ((Case No.: VII ZR 191/12), it is even the case that if this security is not part of the contract, the entire payment schedule is VOID!
That means if your contract does not mention a 5% security FOR you, then you do not have to pay the agreed installments.
Rather, the contractual standard §632a paragraph 1 applies:
(1) The contractor may demand from the client an installment payment in the amount in which the client has gained an increase in value through the service performed as per the contract. Payment of the installment may not be refused due to insignificant defects. Section 641 paragraph 3 applies accordingly. The services must be substantiated by a statement which must enable a quick and reliable assessment of the services. Sentences 1 to 4 also apply to necessary materials or components that have been delivered or specially manufactured and provided, if ownership of the materials or components is transferred to the client at his choice or corresponding security is provided for them.
I would like to post some further links, but the strange rules of this forum do not allow that..