Is "financial low" correct?

I need to translate baja financiera (Spanish) or baixa financeira (Portuguese) into English. That idiom is a noun phrase used to describe a partial or total payment of a purchase or sale.

I'll give an usage example of that idiom. If a client uses multiple payment methods (money, check, debit, credit), every baja/baixa consists of a paid amount and a payment method. If a client pays in multiple installments, every time an installment is paid a baja/baixa is done.

There's a related idiom dar de baja (Spanish) or dar baixa (Portuguese). That idiom is a verb phrase used to describe the action of registering a partial or total payment of a purchase or sale.

I found an English translation for the second idiom: write off. That translation is a verb, but I need a noun to translate the first idiom. I found the option financial low, but I don't know if that idiom has the meaning I expect.

Financial low would not make sense. A low could mean many things, including a point where one has the least of something (financial low ~ point of least money). In English, that doesn't translate to payment. For example, if I'm paying for part of my college tuition, I may incidentally reach a financial low, that is, I don't have much money left in my account, but that's semantically separate from the act of payment or the transaction.

Regarding your usage example:

I'll give an usage example of that idiom. If a client uses multiple payment methods (money, check, debit, credit), every baja/baixa consists of a paid amount and a payment method. If a client pays in multiple installments, every time an installment is paid a baja/baixa is done.

At least in American English, payment would denote giving money by some method. So they might pay in multiple installments, that is, each time they pay a payment is made. You could also say that with each installment a transaction is done. Because translation can be precise, context-specific work, I can't go further than that. I would look up the words in the contexts you're going for to see if they fit or if there are even more precise words or phrases that you'd prefer.