Is one who tells his father a price lower than the real price for something he bought, to avoid angering his father who dislikes high prices, sinful?

The permissible way out in such situations is through allusion (ta'rid) and equivocation (tawriya), for they offer latitude from outright lying. This is done by uttering a word that appears to mean one thing, but another meaning is intended which the word can also encompass, though it is contrary to its apparent meaning. If a prevailing legitimate religious interest or an unavoidable necessity calls for allusion, then there is no harm in it. Otherwise, it is disliked, and it becomes forbidden if used to acquire falsehood or deny a right.