Give every invoice its own number
A number belongs to one job. Do not reuse it, and do not send a new total under a number the client has already paid.
One number, one invoice
Wire Invoice assigns the number when you create the invoice: your prefix, the date, and a sequence. That number is how the client quotes the payment to their bank, and how you find it later.
Do not reuse it
Two jobs under one number become one argument. If you cancel an invoice, leave that number unused. The next job gets the next number.
A changed total needs a new invoice
If the amount changes after the client has the PDF, send a new number. Do not quietly replace a PDF they may already have approved.