T-Mobile US Inc. may not have to pay Sprint Corp. the deal's $600 million breakup fee if the merger fails to get regulatory approval, according to a clause in the wireless carriers' merger agreement. Written into the document is a loophole: T-Mobile (Nasdaq: TMUS) doesn't have to pay if Sprint’s (NYSE: S) credit rating drops below certain preset thresholds before the drop-dead date of July 27. According to the merger document filed with the Securities and Exchange Commission in April 2018,…