Mainz have admitted that Bayer Leverkusen's financial offer and the possibility to sign Marcel Risse on a permanent basis were the reasons they agreed to sell Andre Schurrle.
Germany Under 21 forward Schurrle, 19, will move to BayArena on a five-year deal at the end of the season after an undisclosed fee, reportedly in the region of £6.5million, was agreed.
Several clubs from across Europe were reportedly interested in Schurrle, but Mainz general manager Christian Heidel has revealed that Leverkusen's offer could not be ignored.
Heidel told Sport1 television: "The reasons for our consent are simply economic.
"If there is a club willing to offer a transfer fee in the region of a double-digit (million) price, we are not able to reject that.
"We have to make sure that we do not blow that chance, although we are awfully sorry to do so (sell)."
Part of the agreement will also see Mainz offered the chance to permanently sign current loanee and Germany U21 international Risse, whose temporary stay at Bruchwegstadion is due to come to an end next summer.
Heidel said: "In addition, Bayer Leverkusen have agreed to spare their buy-back right in the loan agreement concerning Marcel Risse."