Since he was to be spared for a while it must mean ultimately exchange or escape.

Santa Anna, a reader of the human face, saw what was in his mind.
"Be not too sanguine," he said, "because I have changed my mind once it does not mean that you are to be free now or ever.

I shall keep you here, and you shall see your comrades fall." A sudden smile, offspring of a quick thought and satanic in its nature, passed over his face.
"I will make you a spectator of the defeat of the Texans," he said.

"A great event needs a witness, and since you cannot be a combatant you can serve in that capacity.

We attack at dawn to-morrow, and you shall miss nothing of it." The wicked smile passed over his face again.