Booking it as a sale on the first payment overstates revenue and understates stock, and it also loses the thing you most need: a clear record of how much each customer still owes and what is being held for them. When the arrangement runs for weeks, that record is the whole point.
Two operational rules that prevent most disputes: hold the specific goods rather than a promise to supply the same item later, and print the customer a document showing what is held, what has been paid and what remains. Verbal arrangements over several months do not survive.
Decide your own policy on the questions the software cannot decide for you: how long you hold goods, whether deposits are refundable, and what happens if the customer never returns. Some countries regulate the terms of instalment sales, so check locally before you write the policy on the wall.