[Lonesome in Massachussets]

the very first written document was probably an IOU

The earliest cuneiform documents were warehouse inventories. Kept the custodian from selling all the goods.

[RoosterRedux]

Interpersonal transactions—exchanges of goods, services, or favors between individuals—almost certainly preceded the invention of warehouses. Early human societies relied on direct exchanges, bartering, and communal sharing long before the development of storage facilities. And interpersonal transactions were most certainly recorded in some manner (to reflect who owes whom).

That said, the cuneiform tablets so far discovered might be the recording of warehouse inventories.