When do you use 'in' vs 'by' when talking about payments and transactions

Question

When talking about buying things and making payments for them etc... what context would you use "in" vs"by" when referring to the payment. For example...

She paid by credit card. (ok)
She paid in credit card.  (not)

She paid by mail. (ok)
She paid in mail.    (not)

She paid by check.    (ok)
She paid in check.     (not)

She paid by cash.    (not?)
She paid in cash.    (ok)

She paid by dollars.    (not)
She paid in dollars.  (ok)

She paid by goats.  (not)
She paid in goats.  (ok)

My sense is that 'by' is used when talking about a method of transmitting value ie: credit card, mail, check where as 'in' is used when talking about the thing of value you are giving as payment ie: cash, dollars, coins, stock.

Is there any relevant literature that talks about this? or is this just idiomatic?

Answer 1

By is used here for the method or the conveyance of funds. Think: How will I be paying? By what means? I can pay by check or credit card (method). I can also pay by mail or by direct debit (conveyance).

In, by contrast, is used for the individual units of the funds. Think: How much will I be paying? In what denominations? I can pay in dollars, euros, or even goats. I can also pay, more specifically, in $20 bills, or "twenties".

Note that cash can actually take either by or in. To pay by cash refers to the use of cash as a medium of exchange; the type or amount of the banknotes are irrelevant. To pay in cash generally refers to a more specific transaction for a more specific amount.

So a catalog might say something like "We accept payments by cash", while someone to whom you owe money might say "I want my money now and I want it in cash!"

Answer 2

in is used for the currency whether it be goats, time, USD, JPY, pounds sterling.

by is used as a means that conveys how the payment was delivered (e.g., by check, by credit card, by working it off)