Find the closest match to a cell , but with a condition.

=SUM(IF(($B$3:$B$23>0)*ABS($A$3:$A$23-$F$1)=MIN(ABS($A$3:$A$23*($B$3:$B$23>0)-$F$1)),$A$3:$A$23))
confirmed with CTRL + SHIFT + ENTER

same can be applied for M:N by modifying ranges accordingly.
(above would return 4800 & 4500 respectively)