It is possible to allow interim payments on a booking-by-booking basis.
Without allowing the guest to pay an amount chosen by the guest, they (the guest) will only be able the amount due at any one point.

To enable the guest to pay custom (guest chosen) amounts do the following:

1. Go into the booking.
2. Tick the box to the right of Custom payment enabled.

3. Click either of the Save buttons.

On the payment page there is now the option for the guest to either pay the amount due. Or an amount of their choosing.