The Gemara Avoda Zara (24b) says:
ת"ש (שמואל א טו, טו) ויאמר שאול מעמלקי הביאום אשר חמל העם על מיטב הצאן והבקר (המשנים והכרים ועל כל הצאן) למען זבוח לה' אלהיך מאי מיטב דמי מיטב
The Gemara further states: Come and hear another objection from a verse: “And Saul said: They have brought them from the Amalekites; for the people spared the best of the flock and of the herd, to sacrifice unto the Lord your God” (I Samuel 15:15). The verse states explicitly that the Israelites intended to sacrifice animals previously owned by gentiles. The Gemara explains: What is the meaning of the phrase: “The best”? This is referring to the monetary value of the best livestock. The intention was not to sacrifice the animals themselves, but to sell them and use the proceeds of the sale to purchase other animals to sacrifice as offerings.
So the Amaleiki animals were to be sold, and other animals where to be bought with the value.