A word for a sale that is not an installment but actually paid in full
Solution 1:
I really like the answers given above. For what it's worth, I suggest lump sum
an amount of money that is paid at one time : a single sum of money
The bonus is paid out in a lump sum.
[Merriam-Webster]
Solution 2:
"Buying something outright" means buying in full without any hire purchase, installment plan, or other arrangement.
Lexico has as one meaning of outright "Not by degrees or instalments." Example sentence: "they decided to buy the company outright"
Merriam-Webster defines it in rather formal terms as "made without encumbrance or lien".
You'll see it widely used in websites on personal finance e.g.
- https://www.drivesmart.co.uk/hpoutrightpurchasepersonal.aspx
- https://www.money.co.uk/guides/what-is-the-best-way-to-pay-for-a-new-car.htm