Instead of killing that Stormtrooper and his informant, Cassian thinks fast and comes up with a different solution. When the Trooper(s) approach, Cassian and drops to his knees, and manages to convince the Trooper(s) that he was sucking Tivik off in the alleyway (and totally not exchanging valuable military secrets). Tivik plays along, and his anxiety about being caught becomes a turn-on for him. The Trooper(s) agree to let them off the hook for public indecency charges... if Cassian's willing to give them the same treatment.

(I honestly forget if there was one or more Troopers in that scene. If you remember, go with that; if not, include however many you prefer. Gender of Trooper(s) is up to you.)