"I didn't buy that" without 了
了 never appears in past negation. 

By definition, one of 了 grammatical functions is to indicate completed action, and not past tense. Hence, if the action did not happen at all (没), it has also never been completed. 


  他还没来 - He still didn't come (and not他还没来了）


A side note to that is that 了 would appear in present negation. 


  他不来了 - He is not coming.