Learn Portuguese (Portugal): Se ele fizer batota, vai pagar caro.

This highlights the current lesson! In Portuguese, certain adjectives can act directly as adverbs (modifying the verb) without taking the -mente suffix. Pagar caro is a set expression meaning to pay a high price or to "pay dearly," and the word caro stays locked in its base masculine singular form.

If the subject were feminine (ela), would it change to pagar cara?

No, it wouldn't change. Because caro is functioning as an adverb here (describing how the person pays) rather than an adjective (describing the person themselves), it is invariable. It will always remain caro regardless of who is paying.

Why is the verb fizer used after se instead of the present tense faz?

When se (if) introduces a condition that is a possibility in the future, Portuguese requires the future subjunctive tense. Fizer is the irregular future subjunctive form of fazer. You would only use the present tense faz with se when talking about established facts or general habits (e.g., "If he [habitually] cheats, I always leave").

Is fazer batota considered slang for cheating?

It is not just slang; it is the standard, everyday European Portuguese expression for cheating at games, sports, rules, or tests. As a bonus vocabulary tip: a person who cheats is called a batoteiro.