In the annuity payment, the interest portion decreases monthly, meaning the interest is always calculated only on the current outstanding debt, while the repayment portion increases.
Since in Germany the interest is calculated monthly in arrears, but the effective interest rate is given per annum according to the Price Indication Regulation, there is always a difference between the nominal and effective interest rate, even if no additional payments other than the interest and repayment rate are made.
Regarding the other matter: Does the bank openly communicate the rejection of the sale of the plot from own funds, that the purchase is only possible if the financing of the house is also included, or is it openly communicated that the plot will only be sold to you if the overall financing is done at the selling bank?
If yes, is this communicated in writing or orally and above all, is this exactly stated in the sales prospectus? Is it also advertised that way?
Basically, it is difficult because the bank cannot be dictated to whom it must sell the plot, and even if there were legal concerns, unfortunately, without taking up the financing from the selling bank, you will not get your plot.