From Leverkusen fan:
According to a German report, Havertz "renounced" part of his salary in order to make the deal happen. Apparently, in his Leverkusen contract there was a clause that saved him 10% of the signing fee when he moves. So he'd get 10% of the initial fee, then 10% of the add ons etc. However, he seems to have renounced on part of his share of the initial fee since Leverkusen and Chelsea struggled to reach an agreement there and instead accepted a higher share of the add ons as compensation. So he's not really renouncing money, he's rather accepting a higher risk to get his share of the add ons but still, he seems to be really committed. The same report also mentioned that Havertz is really impressed by Lampard and his vision for Chelsea.