In page 56 line 13 from the top, of Angelo's Margaris FIRST ORDER MATHEMATICAL LOGIC we read and i quote:

"When an assumption is discharged (or closed to use your terminology)by the deduction theorem it does not disappear; it is transferred across |= (|= is a Logical symbol symbolizing conclusion) to become the antecedent of a conditional "

Hence in our case ~Pa ,when discharged ,it does not disappear , it is transferred further down the proof to become the antecedent of a conditional ,which is :