In fact, it would have been cheaper to have enslaved these Europeans than to spend money to buy and transport black slaves from Africa.
Slavery developed for over four centuries into a major industry especially in the US that, in the 18 century, drove and powered the world economy.
Owners of large properties in the colonial era only wanted profit and would have enslaved anybody to increase their revenue, irrespective of their origin or colour.
If it is a given that the main function of racism is to deny some people equal access to resources, then it is apt to intensify radically when the authority instituted by slavery is disputed, as happened in the United States when slavery was abolished.
The struggle against and later the abolition of slavery did not lead to a race-free social order.