promissory note; written acknowledgment of debt; IOU
A formal or informal document acknowledging a debt, ranging from a simple IOU to a legally binding promissory note.
I lent him money, so I had him write an IOU.
When receiving a loan from a bank, you need to sign a promissory note.
Compound of 借用 (borrowing) and 書 (document). The term is straightforward and transparent in meaning.