3 ideas
Once fa'alavelave contributions, gifts to the aiga and church collections are paid in money instead of goods and labour, each obligation gets an exact figure. That figure can be recorded, compared and chased as a debt. The man who owns the store and extends credit then becomes the creditor of the web of obligation that used to bind people as equals.
A matai title that was once earned through service and consensus becomes something new money can pursue. The new man gets it through strategic giving, church donations and patronage. Once held, the title gives him authority over communal land and labour, which he can feed back into his private business, so status and capital compound each other.
Look at the village church as an accounting system. Public collections read out donor by donor turn piety into a ranked, visible scoreboard of wealth. Families must keep pace or lose standing, so religious devotion becomes one of the strongest engines pulling households into the cash economy.