Hello,
you first have to show the cost framework to the bank when applying. This must be based on an offer and/or estimate from the construction company or architect. A stamp always has to be on it. Even if there are intermediaries in the far north who also obtain such a stamp....
When the loan is disbursed, you usually have to submit a construction status report, possibly photos, as well as invoices. If you do a lot of work yourself, you can also have a flat rate amount paid out and send invoices afterwards (e.g., purchased material).
If you don't have invoices and still want a flat payout of 100k, the bank will have problems at the latest here. After all, it granted a loan for a house XYZ and a construction value of amount 400k and not 300k.
So it will be difficult to park a 100k Porsche in front of the house.
There is some leeway, for example, kitchen and 10k or so. But not such high amounts.
What threatens then is a non-acceptance compensation as well as an adjustment of the loan agreement.