Guest want to send you a deposit for several stays at once, but require billing for each od them separately. In HotelTime 4 you can do this very simply!
Choose document, which should be linked with deposit payment (invoice, reservation, postmaster)
Fill in reservation number (invoce etc.) to link and amount to be used in reservation (invoce etc), from this deposit invoice.
Now you can see in deposit invoice detail attached documents (reservations etc.) and also remaining amount, which can be used in other reservations (invoices etc.)
If you delete the connection between deposit invoice and reservation (etc.) full amount will be available again.