Leviticus 27:27

And if [it be] of an unclean beast
This is to be understood, not of the firstling of unclean creatures in common, which were to be redeemed with a lamb, and not with money, according to the estimation of the priest, and a fifth part added to that; but of such as were sanctified, or vowed, for the reparation of the sanctuary, as Jarchi notes:

then he shall redeem [it] according to thine estimation;
the price the priest should set upon it, how much it was worth in his judgment:

and shall add a fifth [part] of it thereto;
to the price, set upon a fifth part of that over and above the sum; this the sanctifier, or he that made the vow, was obliged to pay, if he thought fit to redeem it:

or if it be not redeemed;
by him, he does not choose to give the price, and the fifth part:

then it shall be sold according to thy estimation;
to another man, without the fifth part, that chooses to purchase it, and then the purchase money was laid out for sacred uses.