Erich-nii-san let me read the letter, too. The content written within was outrageous.
To summarize it: They really didn’t consider Erich-nii-san to be adopted into a family of an equivalent knight peerage household. (ED: Yeah, he married up compared to them!) Furthermore, all the money they had saved up until now was almost completely used for the eldest son Kurt’s marriage in the end. Honestly speaking, they had no money to spend on him.
Moreover, there is no one who would take a congratulatory money to the royal capital either.
Therefore, they left the loaning of money to “you”, is what was written there.