字据 (zìjù): written pledge; written evidenc…

字据 is generic 'something in writing as proof' — a homemade IOU, a handwritten receipt, an informal contract between friends or family. More formal documents are 合同 hétóng (contract), 协议 xiéyì (agreement), 收据 shōujù (official receipt). Verbs: 立字据 lì zìjù = draw up a written pledge; 打字据 dǎ zìjù = write one out.

memory

字 (writing / characters) + 据 (evidence / basis) — 'written evidence.' The 据 here is the same one in 证据 zhèngjù (proof) and 根据 gēnjù (basis); it implies something solid you can point to.