Mark 14:11

And they, when they heard it, were glad, and promised to give him money. And he sought how he might conveniently deliver him unto them.

Mark made no mention of the exact time of payment, but the fact of Judas' returning it that same night shows that there was no long time-lapse, perhaps only time enough for the priests to be sure that Judas would keep his part of the bargain. Regarding the amount and disposition of the thirty pieces of silver and the fulfillment of prophecy connected with this incident, see my Commentary on Matthew, Matthew 26:14.

And they were glad ... Thinking they were then completely in charge of events, they changed their strategy from that of secret assassination to judicial murder.

The strategy of the priests required that Christ be seized when the multitudes were not present; and it was natural that Judas would have counted upon his knowledge of some rendezvous on the slopes of Mount Olivet where Jesus might be spending the night.