Joseph
Ive watched them sell Chametz at MTJ many times (though not in past few years) The way I remember it they sell him the chametz, he puts a down payment and he has x number of days to decide if he want to go through with the sale. Once those days are up he can keep everything forever (and pay the rest of money for it) or the stuff returns to original owners.
There is no “buying back” the chometz from the goy.
Though I am not sure if everybody does it this way. Since I often hear peple refer to waiting for the Rabbi to “but the chametz back from the goy” after PEsach.