"λεπτά" and "λεφτά" are actually the same word. (φ has replaced π in a later evolutionary phase of the word)
They come from the adjective "λεπτός, λεπτή, λεπτό" , which means "thin".
Nowadays, though, these two words are used in a different way (with different meanings).
"λεπτά" means "minutes", "cents" or "thin"
Θα έρθω σε τρία λεπτά. = I will come in three minutes.
Το εισιτήριο κάνει 50 λεπτά. = The ticket is 50 cents.
Αυτά τα αγόρια είναι πολύ λεπτά. = These boys are very thin.
"λεφτά" means only "money"and it is a more informal word than "χρήματα".
Δεν έχω λεφτά να αγοράσω αυτοκίνητο. = I don't have money to buy a car.