There are three Amount fields available in the Field Mapping page:
Vendor Bill folder > Amount
Vendor Bill Expenses folder > Amount
Vendor Bill Items folder > Amount
The Amount field inside the Vendor Bill folder represents the Amount field on the header part of the Bill record. This is automatically being computed by the system.
The Amount field inside the Vendor Bill Expenses folder represents the amount of each Expense line added on the Bill.
The Amount field inside the Vendor Bill Items folder represents the amount of each Item added on the Bill.
Remove the Amount field mapping (the one taken from the Vendor Bill folder) in the Field Mapping column since this is automatically computed by the system depending on the total of Expenses:Amount and/or Items:Amount field(s).