yes, but you can process $500 in purchase first then apply the debit note to purchase, the balance will be $0 and at this moment you can amend the purchase to $100, the balance will be changed to -$400.
I guess this purchase is not the inital purchase, after LZ pay the previous invoices from supplier, supplier sent LZ a new invoice with amount $100 and a debit note with amount $500 for the previous invoices then LZ needs to process them.