MYOB - field :salePrice, "message":"must be greater than or equal to 0.0000","code":"1004"

This means the sell price of a product on the invoice is less 0, the invoice cannot then be synched.

Check each individual line item on the invoice and see if the sell price is set to a negative value if so this item is the problem, change this to 0 or to a positive value.

If the line need to be negative total, the quantity can be negative