Jeff Sessions used political funds for expenses in Cleveland, where he met Russian envoy. Here are some non-paywall versions of the same story: ABC news, and SFgate.
The meeting with the envoy is the one that he lied to congress about having at all, and the one that he said was conducted as part of his job as as Senator on the Armed Services Committee.
It is worth noting that while the meeting itself may have been entirely legal (it is fine for senators to meet with ambassadors) giving false testimony to congress is generally frowned upon. For example, Bill Clinton was impeached over false testimony, not the perfectly legal affair that he was giving testimony about.