Exclude products only on certain circumstances

When a customer orders above €80,00, for example 2 bra's with matching panties, they get the option to pick a free gift. But when they pick one of the panties as their free gift, The 2 panties they pay for change price to €15,00 each. We want the campaign to only work when they pay for 3 panties so the free gift should be excluded. The products should only get excluded when the other condition is met and they become free gift.