When you need to bill a specific amount rather than a scheduled deposit or the full balance, raise an ad-hoc invoice for exactly the figure you want.
Raise an invoice for a custom amount
Open the booking and go to Payment → Invoices.
Click Create Other.
Click the small currency symbol control at the top of the amount field. This switches the field to take a final gross figure.
Type the final gross amount you want to bill, including tax.
Gigwell back-calculates the subtotal and the tax from that figure. It won't add tax on top afterwards, so the number you type is the number your buyer sees.
Check that figure before you save. It's the amount you're asking your buyer to pay, and correcting it after the invoice is issued means unwinding the record.
Covering the rest of the fee
Work out what's left and raise a second invoice for the remainder, so the invoices across the booking add up to the full amount you're owed. Partial payments against a single invoice aren't supported — one invoice per portion of money is the pattern that works.
Worked through with round numbers: on a fee of 10000 where you're billing 4000 now, raise a second invoice for 6000 when the rest falls due.
Good to know
Only the amounts you've actually drafted as invoices appear on the contract. The contract reflects the Payment Plan tab plus existing invoices, so a portion you haven't drafted won't show up on it.
Once an invoice is created, its item and tax fields lock. Get the gross figure right before you create it.
If you're regularly billing odd amounts rather than percentages, set the payment schedule to fixed amounts instead — it's far easier to enter than a percentage that recalculates.