https://www.doujinshi.org/book/753669/

Faris catches Bartz making out with Lenna, and for the sake of her sister's safety offers to sate Bartz's lust in Lenna's stead. This escalates into a bet: if Bartz cums first, he'll never touch either of the Tycoon sisters again. If Faris cums first though, she'll have to bang him whenever he wants. Faris accepts, thinking that she's got the advantage. This doesn't work out well for her.

Not that she needed to bother. As soon as Lenna finds out she jumps into bed, no questions asked.

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This is another work that doesn't take place in the Lenna x Bartz x Faris setting. Though I guess you could call it an extension of Bartz x Lenna & Faris in that it's what would happen if Bartz was your typical asshole hentai protagonist.