Why would he tell The husband that he and her had engaged in sexual activity in order for her to get the letters of transit. What was he trying to accomplish. How did that help the husband and his wife relationship. "Wow she'll sleep with another man for my benefit what a gal".
So that Laszlo would know that Ilsa loved Laszlo as much as Laszlo loved Ilsa and that what Ilsa and Rick had in Paris was long over. Laszlo would have wanted Ilsa's happiness, but Rick knew that Laszlo loved Ilsa so much and that she was as important to his work as the work itself. The whole "hill of beans" thing.
Btw - they didn't have sex in the appt above the Cafe, they both knew though that the love they had in Paris was still alive and well, when Rick was being Rick and not bitter, jaded Rick, which is who he had become.
I also don't believe they engaged in sexual activity. I do think Rick was playing the heel to get Elsa off the hook with her husband. He claims she only pretended, while he was willing to take advantage of her attempts to help her husband.
It was his last shot. He figured that if the Bulgarian husband would have difficulty forgiving his wife if she'd slept with Renault in order to obtain exit visas, then Victor just might feel the same way and decide to leave her behind with Rick. But Rick still loved Ilsa too much to simply blurt it out that she'd slept with him, so he chose his words carefully, claiming that she was just "pretending" (but she still did love him). But Victor was a very forgiving man; he knew what really was going on between Rick and Ilsa, but he wasn't going to let her infidelity destroy him--he wasn't going to end up getting drunk in a bar and calling Ilsa foul names as Rick had. Rick realized that Victor was still the bigger man and deserved Ilsa more than he did.