In arrears: /əˈrɪəz/ if money or a person is paid in arrears for work, the money is paid after the work has been done.
Instalment: /ɪnˈstɔːlmənt/ one of a number of payments that are made regularly over a period of time until something has been paid for. Plazo, cuota. E.g. we paid for the car by/in instalments.
Impulse: /ˈɪmpʌls/ a sudden strong wish or need to do something, without stopping to think about the results. E.g. He had a sudden impulse to stand up and sing. I resisted the impulse to laugh. Her first impulse was to run away. The door was open and on (an) impulse she went inside. He tends to act on impulse.
Hire purchase: /ˈpɜːtʃəs/ a method of buying an article by making regular payments for it over several months or years. The article only belongs to the person who is buying it when all the payments have been made. Comprar algo a plazos. E.g. a hire purchase agreement. We're buying a new cooker on hire purchase.
Credit: an arrangement that you make, with a shop/store for example, to pay later for something you buy. E.g. we bought the dishwasher on credit.
Frugal: /ˈfruːɡl / using only as much money or food as is necessary. Austero. E.g. a frugal existence/life. He has always been hard-working and frugal.