"the very two (the hire of a prostitute and the exchange of a dog)": two and not four (i.e., the hire of a dog and the exchange of a prostitute are permitted) "the very two of them" — they (are interdicted) and not their offspring.
(Devarim 23:20) "You shall not give interest to your brother": This tells me only of the borrower (that he should not give interest.) Whence do I derive (the same for) the lender (that he should not take interest)? From (Vayikra 25:36) "You shall not take from him interest."