"And without doubt the lesser person is blessed by the greater. In the one case, the tenth is collected by men who die; but in the other case, by him who is declared to be living. One might even say that Levi, who collects the tenth, paid the tenth through Abraham, because when Melchizedek met Abraham, Levi was still in the body of his ancestor." NIV translation
Here, we see the ultimate in paying it forward as Paul describes the fact that Levi (servants of God) paid the tithe to Melchizedek as he was an ancestor of Abraham. Melchizedek's priesthood is compared to the Aaronic priesthood in that they were mere men that collected the tithe and, as men, they died but Melchizedek did not. Their ministry was temporary where Melchizedek's is permanent. We often forget that whatever ministry God has entrusted to us is, like Levi's, only temporary but it is for the eternal kingdom and has lasting effects.