SAP Error-Incorrect "Qty (Inventory UoM)" in line 15; change quantity to a different value and press tab key, then change it back to the original quantity

Hi Expert,

We are getting SAP error massage when user want to attach document after posting A/P Invoice only some of them invoices.

Incorrect "Qty (Inventory UoM)" in line 15; change quantity to a different value and press tab key, then change it back to the original quantity

Can any one guide us where is mistake?

SAP version - 9.1 pl-8