This works for your sample data.
Yu

proc sql noprint;
select max(f1),max(f2) into :d1,:d2 from have;
quit;

data want;
do until(last);
set have end=last;
array temp {&d1.,&d2.} _temporary_;
 temp{f1,f2}=v1;
end;
last=0;
do until(last);
set have end=last;
if f1=f2 then v2=.;
else v2=temp(f2,f1);
 output;
end;
stop;
run;