After Shaul promised his daugther to Dovid,he required him to pay the dowry for his daugther this was paid with the foreskins of the Plishtim(Philistines). The question that remains is that there is a Halacha that you are not allowed to gain any enjoyment monetary or otherwise from the body or the parts of a dead Person so how was he able to pay his dowry with the foreskins if it was against Halacha?
The law prohibiting any benefit from the deceased is learned from a long series of gezeirot shavot (a hermeneutical principle involving use of similar words) in Avoda Zarah 29b beginning with the death of Miriam. Because Miriam was Jewish, some only apply this law to Jewish people. Thus the foreskins of a Philistine may be used as part of a bride-price.
I believe the Talmud somewhere discusses of what use is a foreskin, such that it is worth money. It can be fed to a carniverous animal seems to be the conclusion there.