Send/collect first installment before contract period start date

As a coach, I want to be able to send the first invoice or installment, or have it paid, before the contract period officially starts. For example: the contract period starts on 17-08-26, but I create the client on 10-08-26, then I want to be able to send or collect the first installment directly on 10-08-26. Currently only an invoice date on or after the start date is possible.